If MGB is the primary traction motor/generator and MGA is the motor/generator used to extend the Volt’s range by generating electricity when clutched to the gas engine, it would be impractical to use MGA for regenerative braking. That would require clutching it to the drivetrain whenever you took your foot off the accelerator (unless you immediately shifted into N), even in Electric Mode, even if you were moving slowly.While a full stop may be practical in a BEV because there is just one motor, the Volt has a separate traction and generator motor. The generator is only 55kW. Volt has to constantly switch between these motors as the vehicle decelerates/accelerates. So, the regen system is significantly different from Bolt, and not a simple software update.
My understanding is that MGB, the primary traction motor, is used for regenerative braking.