Shimano rear roller brakes come with an arm and fittings to attach it to the chainstay. I imagine that so do Aral brakes. These arms on my Shimano roller brake equipped bikes work well.
There is no reason except expense that you shouldn't have a bracket made to fit the Aral at one end and the disc brake mountings at the other; after all the brake mounts were designed to resolve the torque of a disc brake. Precision marking-out and drilling will be required, the plate may have to have reliefs machined.
Note that the new brake torque reaction arm will be shorter, and very likely flatter, than the factory original, and will thus have to be made of thicker material. The Rohloff Speedbone, which works the other way around (it reflects the hub torque into the disc brake mounts) is a very thick piece of metal, whereas Rohloff's long torque arm intended for fitting to the chainstay is much slenderer.
Andre Jute
