Umm, I would get a separate piece of acetate and 
use a sharpie to draw a mouth on the cel :).  
It's so neat, this year I went to Animazement 
where Watase Yuu was!  I saw a gal there with a 
niiiice Tamahome cel, with no boca!  But, Watase 
drew it on for her!  That was so neat, but, in 
this case, definately do the overlay thing, just 
in case you make a booboo ^_~