I've never come across cels backed with glassine, 
though it does sound like it might be a good 
idea, given that stamp collectors keep their 
stamps in glassine envelopes to prevent sticking.

I've seen quite a few cels backed with what I 
believe is rice paper.  I'm pretty sure it's not 
glassine because it's slightly pourous and 
weakens in water.  Glassine on the other hand is 
sturdy and quite resistant to air and water.  If 
a cel somehow got stuck to a piece of glassine, 
it would be a real challenge to remove it.