I think you want to use the control loop and command a
lower speed than rotor speed. You have to respect the
battery's charging current limitations and you want some
proportionality if this is a transportation application -
who brakes by locking them up?
If you can command negative torque, this is your best
regen braking I think. But you do want the energy to go
back to the battery instead of ground for regenerative,
so I think that wants a boost capability. Maybe some
BLDC drivers can operate that way but I would not
assume that of them all, especially the cheap ones.
Maybe simpler to attach a rectifier pack and a boost
converter, current-mode, than to try and retrofit the
function to an existing design.