Here is a list that was created in the old forum and should be accurate up to the end of that forum... Obviously it doesn't include EPs that came out since and I'm sure there are some that are missing from the list. It's also a shame that no one put the entire thread here on the new forum...
Works:
Ernie Ball VP Jr 25k ohm (log pot)
Line 6 EX-1
Roland EV-5
Boss FV-500L
Boss FV-500H
unknow needs confirmation:
Kurzweil CC-1 (10K linear pot, 45 bucks, likely compatible)
TC Electronics X1 (25k ohm Logpot, $130 (!), likely compatible)
Pigtronix EP-1 Expression Pedal (20k pot unknown taper, metal construction, around 50 bucks, likely compatible)
Not Compatible:
Yamaha FC-7
Ernie Ball VP JR Passive 250K & 6166
Any wah wah pedal
Morley volume Pedals
Behringer FCV100
M-Audio EXP
Rocktron Hex
Moog EP-2 (it is possible to swap the internal pot with a 16mm pot, though it might take some fabrication skills. This is not recommended for the inexperienced.)
Hope this helps!
Please note: I cannot take any responsability for any discrepencies on this list. I did not write it and I did not test any of the components on it (except the Ernie Ball VP JR 25k ohm, wich does work)