Good news, everyone!
My attempt of inserting G&W's model into Mario works! ...sort of.
You can't appreciate it here, but he's in 3D with all his weapons out.
It's kinda clearer here.
All taunts and attacks work. Here's the up-taunt...
...Neutral B...
...and Down B. It always shoots directly upwards.
Next, I'm going to try Mario's model on G&W and G&W's model on Jigglypuff.
EDIT: Results are here!
Mario on G&W: Game & Watch loudly scolded me for replacing him with Mario (aka BEEEEEEEP).
G&W on Jigglypuff: Dissapointingly similar results to the first one.
3D with all his weapons out...
Better pic...
Neutral B...
Down B...
Now, this one was weird. He got thrown to the ground, this appeared and the game lagged a lot.