Please keep us informed on how you do it. I think the optical route is definitely the best way. I will do one when I build my Megadrum. The Alesis module that I have doesn't support a variable hat.
Dmitri, do you have a schematic for the one you built? This thread has got me interested. I had read about that optical controller on the eDrum website, but like elrules I wasn't sure if it was variable or not.