I haven't studied the Platonic influence on Eucharistic theology, but you are quite correct to see that Augustine did not believe in transubstantiation. There is no question that Augustine believed in a "real presence" of Christ in the Eucharist, but this is in the faith of the believer, and not in any of the physical elements of the material itself. This understanding of the Lord's Supper was transferred to the Reformed faith through John Calvin and is known as "Suprasubstantiation," understood in this way:
"Christ is present in heaven but by virtue of the Holy Spirit our souls are lifted to partake the flesh and blood of our Lord. Our mouth eats the sign (bread and wine) whereas the mouth of faith eats that which the sign signifies, namely, Christ and the benefits of the covenant of grace" (Hyde, Daniel. In Living Color: Images of Christ and the Means of Grace. p. 154. Grandville: Reformed Fellowship, 2009.).
In other words, eating with teeth and stomach effects nothing, but it is a spiritual act that is performed through faith. Augustine frequently speaks using "realistic" language, but, in the same writings, makes it clear that he is speaking spiritually, and not materially. Note the following quotes which reflect the position of the quote above:
They said therefore unto Him, What shall we do, that we may work the works of God? For He had said to them, Labor not for the meat which perisheth, but for that which endureth unto eternal life. What shall we do? they ask; by observing what, shall we be able to fulfill this precept? Jesus answered and said unto them, This is the work of God, that ye believe on Him whom He has sent. This is then to eat the meat, not that which perisheth, but that which endureth unto eternal life. To what purpose dost thou make ready teeth and stomach? Believe, and thou hast eaten already. (Augustine, Tractate 25)
Believe, says Augustine, and "thou hast eaten already," also from his commentary on John chapter 6. In the next chapter of his Tractates on John:
Wherefore, the Lord, about to give the Holy Spirit, said that Himself was the bread that came down from heaven, exhorting us to believe in Him. For to believe in Him is to eat the living bread. He that believes eats; he is sated invisibly, because invisibly is he born again. A babe within, a new man within. Where he is made new, there he is satisfied with food. (12) What then did the Lord answer to such murmurers? Murmur not among yourselves. As if He said, I know why you are not hungry, and do not understand nor seek after this bread. Murmur not among yourselves: no man can come unto me, except the Father that sent me draw him. Noble excellence of grace! No man comes unless drawn. There is whom He draws, and there is whom He draws not; why He draws one and draws not another, do not desire to judge, if you desire not to err. (Augustine, Tractate 26)
To believe on Christ, says Augustine, is to eat the living Christ.
In another quote, the Eucharist, which symbolizes both the entire church and Christ, not really consumed. The Eucharist signifies an invisible reality, and is not that reality. Christians should take the spiritual lesson of unity from the Lords supper.
What you can see passes away, but the invisible reality signified does not pass away, but remains. Look, its received, its eaten, its consumed. Is the body of Christ consumed, is the Church of Christ consumed, are the members of Christ consumed? Perish the thought! Here they are being purified, there they will be crowned with the victors laurels. So what is signified will remain eternally, although the thing that signifies it seems to pass away. So receive the sacrament in such a way that you think about yourselves, that you retain unity in your hearts, that you always fix your hearts up above. Dont let your hope be placed on earth, but in heaven. Let your faith be firm in God, let it be acceptable to God. Because what you dont see now, but believe, you are going to see there, where you will have joy without end. (Augustine, Ser. 227)
The body of Christ not held by any believer, even in the sacrament. Christ is held in the heart, and not in the hand. This cannot be so if transubstantation is true.
Let them come to the church and hear where Christ is, and take Him. They may hear it from us, they may hear it from the gospel. He was slain by their forefathers, He was buried, He rose again, He was recognized by the disciples, He ascended before their eyes into heaven, and there sitteth at the right hand of the Father; and He who was judged is yet to come as Judge of all: let them hear, and hold fast. Do they reply, How shall I take hold of the absent? how shall I stretch up my hand into heaven, and take hold of one who is sitting there? Stretch up thy faith, and thou hast got hold. Thy forefathers held by the flesh, hold thou with the heart; for the absent Christ is also present. But for His presence, we ourselves were unable to hold Him. (Augustine, Tractate 50)
One final quote for now. Augustine speaks of the use of "common parlance" (iow, "realistic language) such as saying that Christ is raised tomorrow "On Easter," though not literally, and that the bread is the body of Christ, in the same sense as the Easter example:
You know that in ordinary parlance we often say, when Easter is approaching, Tomorrow or the day after is the Lords Passion, although He suffered so many years ago, and His passion was endured once for all time. In like manner, on Easter Sunday, we say, This day the Lord rose from the dead, although so many years have passed since His resurrection. But no one is so foolish as to accuse us of falsehood when we use these phrases, for this reason, that we give such names to these days on the ground of a likeness between them and the days on which the events referred to actually transpired, the day being called the day of that event, although it is not the very day on which the event took place, but one corresponding to it by the revolution of the same time of the year, and the event itself being said to take place on that day, because, although it really took place long before, it is on that day sacramentally celebrated. Was not Christ once for all offered up in His own person as a sacrifice? And yet, is He not likewise offered up in the sacrament as a sacrifice, not only in the special solemnities of Easter, but also daily among our congregations; so that the man who, being questioned, answers that He is offered as a sacrifice in that ordinance, declares what is strictly true? For if sacraments had not some points of real resemblance to the things of which they are the sacraments, they would not be sacraments at all. In most cases, moreover, they do in virtue of this likeness bear the names of the realities which they resemble. As, therefore, in a certain manner the sacrament of Christs body is Christs body, and the sacrament of Christs blood is Christs blood. (Augustine, Letters 98)
There are many more quotes that establish Augustine's position on the matter. It is clear that Augustine did not hold to transubstantiation, but to what the Reformed know as "Suprasubstantiation."
Came across this (not to say ECFS were always consistent with themselves, much less each other - or Rome - or could not hold compatible opinions);
Jerome: The Body and Blood of Christ are Poured into our Ears
We have read the Sacred Scriptures. I think the Gospel is the body of Christ; Holy Writ, His teaching. When he says: He who does not eat my flesh and drink my blood, although the words may be understood in their mystical sense, nevertheless, I say the word of Scripture is truly the body of Christ and His blood; it is divine doctrine. If at any time we approach the Sacrament the faithful understand what I mean and a tiny crumb should fall, we are appalled. Even so, if at any time we hear the word of God, through which the body and blood of Christ is being poured into our ears, and we yield carelessly to distraction, how responsible are we not for our failing? FC, Vol. 48, The Homilies of St. Jerome: Vol. 1, On the Psalms (at Psalm 147), Homily 57 (Washington D.C.: The Catholic University of America Press, 1964), p. 410.
http://voxpatristica.blogspot.com/2013/02/jerome-body-and-blood-of-christ-are.html