on the simple basis that I hate people saying its impossible, I'm posting
its not impossible and its not perfect but as I say, its doable.
I did it here:
http://emutalk.net/showthread.php?t=29150&page=2&highlight=backyard
scroll down to the last post for a more visible illustration of it done, its just a shame some posts like that get ignored because its not zelda/mario or SSB.
and I've done it on Zelda (put the triforce on the pedestal thing, but pictures evade me.)
All you do is edit the models code, point by point. The models are called in and stored as vertex data, luckily, so near the end of the code you get model data. Its a case of finding whats what. Data is stored XXXX YYYY ZZZZ ---- --JJ KKLL RRGG BBAA
Where J, K, L are X, Y, Z on UV mapping co-ords respectively, R, G, B, A is Red, green, blue, alpha on colour data and X, Y, Z relates to its physical space.
I THINK thats it, from best of my memory. I know XYZ and RGBA are presented like that although I think RGBA might have been padded at the end or something. I dont know, its been years but you'll easily pick it up.
THEN, you just need to edit the model bit by bit (very long but you get to eventually see it in your head point for point. Its a bit like the matrix, haha.)
To put your edit in the game you can either use a lot of Gameshark instructions (normally,"if Xmodel is loaded at Xpoint, this line is written") but me and a mate were working on a recoding tool that'd read and write to pointers. But we both lost interest after a while, it was only No Mercy and we've got jobs to work to.
So basically its not impossible, don't say that it is, but its long winded and will require a little bit of extra innovation from the maker. Good luck
(edit: I did have a help with No Mercy given that I knew where the actors were loaded, and knew where their pointers to model data were. This is no big deal - you can easily find these in any other N64 game. I found them in Zelda, Mario... all N64 games are the same as they ran off a unified gfx code.)