Have you seen those special cut paper punchers 
for kids that will punch star shapes or Mickey 
Mouse shapes, etc?  I've seen more of those out - 
only made for office use now - which will punch 
words or initials. You might be able to get one 
of those made to say "fan" or somesuch, and that 
might be even better than a signature. First, 
since it's a punchout and not writing, it's near-
invisible for when someone is looking at the cel 
to enjoy the cel. But it's also very not 
removeable (especially if punched near the image) 
for if the person goes to resell.

Also, I think I remember them being fairly cheap 
to have made. (We get cataloges like that sent to 
where I work and that's where I saw the office 
ones.)

Many Sharp Smiles,
--Drac