Sounds like what you really need is a high-frequency low-bitdepth DAC, and a high-bandwidth comparator, and then "play" your signal out the DAC.
With something like a 192 kHz 8-bit DAC, you should be able to feed it through SPI fast enough to drive the motor and do some control on the CPU. More than 1 byte of buffer depth would help, though... You may need to look at a higher-end CPU in the end.