I've seen that cel for auction on eBay more than 
once.  The first I remember seeing it as was 
a "Doujin Cel", meaning it was made by an artist 
(which you know already).  They said it was by 
someone in Japan.

Whether or not it could be a hanken, perhaps from 
a hentai game, I do not know.  But personally I 
feel it is just a fancel made from a very 
talented artist in Japan.  

I'm curious, too, though.  Anyone else know more 
about her?