How do I determine the power rating of a stepper motor?
I would suggest driving the motor with 72% of rated current, or about 310mA.  That should yield the same power dissipation as would be expected from unipolar drive at 440mA.  When the motor isn't moving, the voltage would have to be about 31 volts.  I wouldn't expect insulation problems at 31 volts, or even 50, but I don't know how far one could safely go.  Perhaps measure the AC voltage when hand-spinning the motor quickly to see if there seems to be a limit.