The Fly rack uses a strut and an attachment that shares the same hole as the brake mounting bolt, it doesn't mount on the brake caliper itself. You will have to have a way to anchor the bottom part of the rack, either on your seat stays or the quick release.
If you haven't already purchased the rack, may I suggest you talk to this guy:
http://www.thetouringstore.com/TUBUS/Fly/FLY%20PAGE.htm really knowledgeable and helpful, particularly when it comes to situations like yours.