I can think of two gotchas:

1) Is there an eyelet in the same place on the drive side? If not, your rack will mount all crooked.

2) You'd have to mount the rack support on the outside of your disc caliper (bolt head > rack support > disc caliper > frame) so that you don't mess with the spacing of your caliper.

Most rack makers offer models that are specifically made to be disc-compatible, often with L-shaped rack supports that can clear the caliper.