Could you use a frequency comparator and a bang-bang limiter? You can use the phase/frequency detector and the VCO in a CD4046 for this purpose. The output of the comparator would simply shut the motor off if the speed was above the preset limit, and allow power to be reapplied (with whatever proportional control you have) when speed fell below the limit. Do you think this would be too jerky? If you let the motor coast, as opposed to shorting it out, it might be OK.
I think this would be simpler than an LM2907. The LM2907 approach has lag due to the filtering required for the f-v conversion process. I have to confess that I haven't used the LM2907, but I do know that in order to have low ripple at low speeds (say 1 mph, ~ 4 Hz), the filter time constant is going to be large, probably greater than 1 second, which will, I think, also be the lag. This may be OK, but you know more about that than I do.