We use brake mode to hold our robot up after we climb, but I’ve never actually seen a list of values for holding torque for motors in brake mode to reference. This seems like it could be a useful thing, even if it’s minimum output approximations.
I don’t know of any general measurements. In any case, the gearbox will also provide some braking force beyond multiplying the braking torque. At this point, the best answer is to measure/test it yourself.
In general, torque applied by brake mode will be proportional to velocity (i.e., it acts like a dashpot), so there’s probably not a good constant value that you can use. Any holding torque provided at zero velocity is likely due to some miscellaneous stiction, which is pretty inconsistent and hard to predict.