Andy hinted at this, but are you going to have diagonal stays? Put the brake there. The only issue might be finding a long enough straddle cable. That could be solved with cantilevers.
A good bike shop should be able to help you with a long straddle cable though. Mafacs had an adjustable straddle cable on their centerpulls, and they are probably the best of the old-school centerpulls.
On edit: there are cx bikes with pulleys for the front derailleur, so you should be able to find one of those and then fabricate the rest.
Last edited by unterhausen; 11-24-21 at 10:34 AM.