It's not so much a reduction in the voltage required to keep it latched, but the current.
No one can say how low the PWM freq can be, for that we need to know the inductance of the coil, the coil resistance, and the forward voltage of the flyback diode. BTW, you know you need a flyback diode in there, right? That's not just to suppress high voltage spikes but it actually sustains the magnetic field in the inductor and reduces the required period.
If you get the latching current too low, it may be prone to unlatching if bumped around since the coil's only putting out slightly more force than the return spring.