To hold a servo motor at constant torque, all you need to do is provide a constant voltage. You can accomplish this using the analog output (DAC) of a FlexMotion board. To gain access to the DAC, configure the axis as an open loop stepper, then use the Load DAC.flx VI to output an analog voltage. The downside to this is that you won't have feedback, so you'll have no way to determine your position.