Judging from the way the rack stays are bent, it looks to me like the seat will only work with the specially-designed Topeak rack. I have 2 other Topeak racks (not disc compatible, though) that don't look anything like that.
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.