Wasn't there a 3rd party Macintosh clone (early laptop?) that used a standard PC drive modified for variable spindle speed? That might turn out to be fairly straightforward depending on how the spindle motor speed is regulated, but you'd have to consult the datasheet for the motor control chip used in the particular drive you wanted to modify.

That said, if I were undertaking a project like this, I would definitely stick to fixed speed 1.44 disks to begin with. I suspect there will be timing difficulties and a lot of screwing around required to get anything working reliably. Bigmessowires' Mac floppy emulator would be a good starting point to look at.