Mounting long reach center pull brake on Marrakesh (or similar disc-braky) frame?
As Sheldon Brown notes, 650B rims are 19mm further away from the brakes than an equivalent 700c. Given that your frames were designed for disc brakes in the first place, you will have some difficulty in finding rim brakes with enough reach. You might need an unsafe hack -- especially if you are using fenders. 

Furthermore, your brake cabling is set up for disc brakes so you'll need to get new braze on guides or use ugly zip ties. 

Most people would wonder why you'd go to such lengths to have worse brake performances and worse looks in order to keep your hubs. There are a number of great looking disc brake hubs now available. If you found one with the same spoke count, you could keep your rims.