I know freezing - or even just cold weather - works, because
I had a cel sent to me in winter that was stuck to the 
background at the dealer's, but was unstuck by the time I 
got it!

However... why bother removing rice paper?? To be honest, I
bought some rice paper to back all my cels, because I would
rather have it stuck to something transparent than to 
something opaque, like a sketch or background, or even to 
the cel book/bag or, heaven forbid, another cel.  I know its
all a matter of personal taste, but I don't mind my cels 
being stuck to rice paper, but I DO mind them being stuck to
anything else, so I just put the rice paper there on gp.

Az