I think the important question is this: if Mangini, for some reason, was unable to continue playing in DT and they held auditions again, would Portnoy have to go through the audition process?
This won't be a first, since Ian Gillan had to practically audition for Deep Purple when the 3rd (and final?) reunion of the famous Mk. 2 happened, back in 1993.
But, if MP ever returns (and that's a big if), his role will be completely different.
And to answer the initial question, no, I don't see MP learning the parts because a) he probably won't be able to and b) if a reunion happens, they will have a lot of material to play live from the MP albums, so no need for the MM songs to be played. Jon Anderson did it, Ian Gillan did it, Ozzy Osbourne did it.
As for clinks63, I have to remind you that most of the times in the history of music, it's been proven that the sum is greater than the parts, so DT=MP isn't that accurate at all.