I have a lot of Topeak stuff and they seem pretty fond of their rack system, so I'm guessing if the seat comes with a rack it's pretty much designed only to work with that rack.
Can't tell if you've tried this already, but I'd suggest you use the REI web site to find the nearest store (even if it's not that close) that has one in stock, and then call the store directly. I've always found REI folks more than helpful. I've never gone so far as to request cell phone pictures of anything, but it wouldn't hurt to ask!
As a last resort, you could order one from Amazon or some place with a generous return policy and check it out yourself.