Motor Power Limiting

Is that a typo? How can you have a 50W power rating for a 24V motor with that high of expected currents?

Are you sure the 50A isn't a short circuit current and your rated current at load is only 4-5 Amps?

The maximum continuous current is only 2.85 A. The starting current is given as 50A, but if I understand correctly it is only possible to provide this for a short period of time, otherwise the motor windings will overheat. 

Your comment doesn't make sense: " I want to supply the stall current, without exceeding the motor power rating of 50W."  In fact you DON'T want to supply the stall current at full load.  You should limit this to the rated full load current.  Else you can brake the motor in this case (turning on either all high-side or low-side switches in the inverter).

When the motor is running at low speeds, it can be overloaded for a short period of time.

I now understand that the stall current depends on the PWM modulation index. The motor will only draw 50A if the modulation index is 100% (24V being applied from phase to phase). From a 50W supply, the maximum current that the motor will draw is 10A at stall (5V from phase to phase). This can be predicted from a speed vs. torque graph. 

If the load requires less torque (equivalent to 7A), then it can spin up to 20% of it's maximum speed, without drawing more than 50W from the supply. The motor cannot draw 7A indefinitely, because the power dissipated in the motor windings = I²R. If the enclosure cannot dissipate the power from the windings, then it will overheat. 

I'd like to run the motor in this overload condition, even if it can only be done for short periods of time. I need as much torque as possible. The motor in my application cannot be changed, but it does not run for a long time, so overheating is not a concern. Setting the maximum output of the speed PI controller to the rated full load current is suitable for continuous operation, but it will not permit the motor to enter the overload region. Are you aware of any control scheme that allows for this?

The overload region on the speed vs. torque graph is labelled as "occasional use" on the graph below. The image was sourced from http://www.phidgets.com/docs/DC_Motor_and_Controller_Primer.