Have you thought about using an Eddy Current Brake... mount onto the motor shaft, an aluminium disc; the diameter is
determined by the amount of breaking required, along with two fixed magnets, or electromagnets; these are set
between the disc outer rim area = N to S pole fields through the disc, set as close to the disc surface as possible...
With electromagnets you could vary the field strength, and alter the applied load; heat would build-up within the disc
and be dissipated in-air and through the motor rotation... this system was used by Philips in there video recorder,
to engage the tape spools, by shifting the motor/magnets into the spool discs.
Some experimental work or testing would be required; otherwise see Wiki:
http://en.wikipedia.org/wiki/Eddy_current_brake