Ben_H, topeak do a disc rack for 29ers which has l-shaped bits at the bottom that clear most disc brakes: am looking at this for the caadx (if it ever turns up at the shop!), and salsa do lovely seatclamps with top rack mounts welded on the back.
Thanks julianwilson.
I tried the Topeak rack – it fits fine on the drive side, but fouls the brake side because the mount is placed so close to the dropout cowl (which protrudes and prevents the square end of the Topeak mount getting close enough to the mount). I’ve also tried a QR-axle adapter from Tubus and had the same problem!
The more I think about it, perhaps I’m wrong in my interpretation of them being lower rack mounts (maybe they’re just mudguard mounts).