I believe that Catholics understand the "Lamb that seemed to have been slain" which stands before the throne in Revelations 5 to be Jesus eternally presenting His Sacrifice of the Cross. So while Jesus continually presents in Heaven the sacrifice, so too is it continually presented in tandem on Earth in the Mass.
Nowhere does the text of Revelation 5 indicate that Christ is "eternally presenting His sacrifice." It simply says that they see a "Lamb as it had been slain," ie He was recognizable as the one who had made the sacrifice. Seems to me to be a bit of a stretch to suggest otherwise.
In fact, look at the tense. These events are taking place in heaven, which can rightly be described as outside our time, and yet in the song the Lamb is said to be one who WAS slain, who redeemED them to God by His blood, who has MADE them kings and priests. Here, in the presence of God, the event is spoken of in the past tense, as something that has already taken place. It's not stated that He is being slain, is redeeming them, is making them kings. It has already happened, as I said, from God's perspective.
Another thing to note - Jesus has two roles, also described in that chapter. He is not just the Lamb of God, who came to take away the sin of the world. He is the Lion of the Tribe of Judah, coming to exact judgement on the earth. How could He be perpetually, eternally offering the sacrifice of His death when He has a different role altogether to yet assume?