I have the "save" functionality working now and it too appears to be compatible with the original interpreter. Haven't done much testing yet, but I was able to load a game saved in my interpreter in to the original interpreter. No doubt there will be some broken bits here and there that I'll discover when fuller testing is done.
I'm now looking at the simple save functionality. I think I already have that functionality in place, but when I tried it with Mixed Up Mother Goose with a clean slate, i.e. no pre-existing saved games, it asked me to pick my name from a list that was empty, and the pointer was able to move up and down over the whole screen rather than just within the empty list.
Turns out that the original game did this as well, which I found quite strange. I'm using 2.915 of the interpreter. Can others that have the AGI MUMG give that a go as well, i.e. try firing up the original AGI interpreter with MUMG and no saved games and confirm that there is a bug there with that initial name selection dialog?