I have the Vega, which is pretty close to the Fly, except it mounts to rack bosses. Racks have been attaching to brake bridges for a long time. I just sold a vintage Blackburn from the 80s that attached like so. I wouldn't worry. I mean, I wouldn't strap 60 pounds of stuff on a Fly,

You can't go wrong with Tubus. I just wish I'd bought the stainless version.

Wayne at The Touring Store has a nice writeup. I hear he's excellent to deal with, also. http://www.thetouringstore.com/TUBUS/Fly/FLY%20PAGE.htm