[extrapickles]

Most EVs have permanent magnet motors of some flavor right now (some have both a PM motor and a pure induction). I would assume that if you were having the motor be the primary braking system, and it was induction, there would be a tiny cheap magnet installed in it to generate bootstrap power to prevent exactly this scenario.

[londons_explore]

An even simpler design is to assume that the car can only start moving when there is power. If there is a loss of power while moving, then you can use the energy in the HV capacitors in the inverters to make a bootstrap field. That can be maintained till the car stops (and the user still has control of when/how much to brake). When the car comes to a halt, the parking brake is applied, and it won't move again till repaired.

All of that can be done with software-only - no hardware changes needed.