This' a very interesting idea. Is there enough space in the rom for it?
Unless I'm mistaken, it actually wouldn't take much space on the ROM at all. The largest chunk would be encoding names for the endings, and that could be shortened by just numbering them instead. It would just need a bit of code to play certain chunks of event code before returning to the ending player menu, as well as an in-game trigger to get to the ending player. And possibly a few flags, if the endings were unlocked in the viewer as you got them, rather than just all being playable by default. The problem with the flags would be that would require a change to the save file format, which would likely make all saves from versions without the ending viewer not compatible with saves with the viewer.
Honestly, the best place to put the ending viewer would likely be somewhere in the "secret" developer's room (probably replacing the text of the guy that tells you "can you find all 12 endings?", when there's actually closer to 17 distinct endings, plus variations of a few of those), and if it's in the Dev Room, then it makes sense that you'd be able to view all the endings, even if you hadn't viewed them previously.
It would be cool if it let you play the different versions of the endings, such as the differences from having saved Lucca's mother or not affecting if she's in the parade ending, and such; but that would require another
chunk of code to set various flags, and probably a third chunk of code to get the events to pay attention to that set flags rather than the ones in your save...