Here is a supported and non supported list from the support archives:
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.)