So the regen force would still exist (?), just it would be really small, and so taking a long time to fully stop. Would it be possible to apply a reverse torque to the electric motor to decelerate faster. For example, I'm thinking about a hypothetical race car that could have no traditional hydraulic braking system to save weight, a large amount unsprung.
No. The force provided by regenerative braking is proportional to your speed. Without friction, it would be impossible to reach 0 speed.