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Is it something I can work out mathematically
It is, but you don't need to, if you use same pick up point as the roller brake used. They will have built the frame to accommodate as much torque as the tyre can generate.
The maths is coefficient of friction (assume 1.0 for rubber on tarmac) × normal force (i.e. the weight loading on the tyre) × contact radius (half the overall diameter of the tyre). For the rear you can assume the normal force is the static load, typically 50-70% of all-up weight, but for the front you should assume that the tyre is bearing 100% of the all-up weight because under braking the load transfers fully off the back tyre.
Sample:
150kg all-up, 70% on the back tyre, on Earth rather than any other planet, 700×40C tyre
1.0×0.7×150kG×9.8N.kg-1×0.35m=360Nm
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gbj_tester
Thanks Tester. Your answer leads me to another question, which is how I can ascertain the brake torque reaction for this frame. Is it something I can work out mathematically, or do I need to write to the manufacturer?