Ive seen many ppl using PLL for DC motor control but why not use a PLL for an ac syncro motor ? Throw a crystal on the input. Use filters to clean up the VCO's output, followed by a nice large BJT output stage to drive the motor. Including the filter and output filter in the PLL's feedback loop wil miminize distortion and phase shifts. You can also throw in a multiplier and programmable divider and you could have very fine adjustments of the oscillator frequency. Just some random ideas of mine. Seems simple enough to me, any reason why this wouldnt work ?