Is it possible to decrease the power dissipated in a winding with a constant braking torque?
The motor heating goes as the winding current squared, which goes as the braking torque squared. So your motor heating will only allow a certain braking torque. If you only want to brake for a short time, you might allow the motor temperature to rise to absorb rather than dissipate the heat, subject to long enough being allowed between braking events to allow the motor to cool.
The motor voltage will depend on speed, so when braking from high speed you would be able to use an external load to dissipate some of the braking energy. At some lowish speed, the external load would need to be zero to allow enough braking current to flow. Below that speed you would have to actively drive a braking current into the motor.