There are exceptions.
Our parish has adopted a set of missions in Bolivia, and the nun who administers them comes up once a year to give us an "update", and it's done during the homily. Usually very inspirational.
We also just completed three "Stewardship weekends." On one of them, a layman got up and detailed how he came to become involved in, in his case, the lector program and taking the Eucharist to the homebound. He did this during homily time.
"He did this during homily time."
What is wrong with the time right before the blessing at the end of Mass?
The GIRM makes no provision for this kind of a report at the time of the homily.
65. The homily is part of the Liturgy and is strongly recommended,63 for it is necessary for the nurturing of the Christian life. It should be an exposition of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or from the Proper of the Mass of the day and should take into account both the mystery being celebrated and the particular needs of the listeners.64
66. The Homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person.65 In particular cases and for a just cause, the homily may even be given by a Bishop or a priest who is present at the celebration but cannot concelebrate.
This is of course the standard and for good reason. The rubrics are broken always in the name of a "good cause". That is always the justification for disobedience. It's as American as Huckleberry Finn. I do not dispute the goodness of the cause, merely the timing for the pronouncements of that cause. There is no need for disobedience to the GIRM. You have used it to defend your own positions on this forum and rightly so. The American Catholic Church has selectively disregarded those portions of Sacrosanctum Concilium and the Girm which apparently lack the sophistication due a progressive-oriented liturgy.
I cannot see why at the end of Mass the priest cannot simply ask for some more time for the inspirational speeches from the nun outside of the Mass.