Just a thought...that acidic paint-eating uhm, 
paint that gave you grief with your Sakura and 
Fish cel...

What would happen if you painted a signature on 
the cel with that, apart from the image but just 
partially under it?  I'm thinking of the etching 
acid I use for glass craftworks - maybe this could 
permanently mark the cel in a distinctive way so 
that people can tell it's a fan cel, even if 
someone tries to obscure it.