[Tamaki-Kun]

So long as:

1. Custom sprites don't cause enormous save file bloat/lag issues

and 

2.  A very up-front set of rules is created and shared prior to their introduction to avoid abuse cases

Then I see no way this can do any harm, and no reason not to implement it as a Patreon reward. 

Creative examples of abuse cases would be: 

• Spriting a weapon to look like Kael tail+ears to fake character race.
  
• Spriting a weapon to make the character look like the legendary Blue-Eyes White Dragon.
  
• Wielding a sword that's actually just Sans, Dick Butt, or some other terrible meme.
  

Yeap. I imagine the GMs are going to be watching over all of us from high above in regards to it. I mean, item regalias already exist and I'm certain they keep an eye on those too. In fact, this'd probably(?) be easier to regulate considering it's visible on our sprite.