As sranck said:
I think I figured it out. Pri Axis is the X axis, and Sec Axis is the Y axis. Given those two, the Z can be computed internally. I use Make Rotator and then get its Forward Vector and Right Vector, and plug those into Set Constraint Reference Orientation, where Pri Axis = Forward Vector, and Sec Axis = Right Vector. Then, I can simply plug values into the Rotator’s Roll, Pitch, and Yaw to orient the constraint relative to the frame of reference it’s attached to.
Pri Axis is Forward vector, Sec Axis is Right vector of frame.