15th Character Workaround

It's highly doable, I'd say check out this video here to get an idea of some trials and tribulations with it:
https://www.youtube.com/watch?v=xcOQEjkqtG8

It sounds like your aware of most of this though. Besides this, there are little things like needing to update the unequip NPC to include his character ID, and other event related issues that might come up, where they do not program any character to be on the screen higher than Umaro (0D). That's the only things that come to my mind atm. The main issues are with him in the Colosseum and I think the final battle roster list, but referencing Hatzen's Guest Adder patch code can probably help solve those issues.