The big thing about putting motors on the belt drive is that you may have to change how the belt is tensioned, depending on its path. Conventional belt drives are designed around the engine generating positive torque, and the accessories generating negative torque.
The moment you have an accessory generating positive torque, you have to consider that in your tensioning system, and that makes retrofit belt-drive hybrids very difficult. (Note that XL Hybrids does have a system that mounts on the output shaft of RWD pickup transmissions, and that seems to provide noticeable benefits in city driving.)
Additionally, you have to control this thing. How do you determine when to regen, and when to boost? A naïve approach would be to simply regen either whenever 0% throttle is requested, or whenever the brakes are applied (note that this creates very inconsistent brake feel if your regen is binary!), and boost at certain throttle settings (a very naïve approach would be to boost at low throttle... this may actually hurt efficiency by encouraging low engine power settings! A better naïve approach would be to boost at high throttle).
(Worth noting that production hybrids use hybrid power at low power settings... but the engine is off. That greatly improves efficiency if you can do that, but a retrofit system can't easily do that, and it definitely can't if it's on the accessory belt drive.)
And, you have to interface all of this with the rest of the vehicle systems, of course. Easiest way would be to intercept the throttle and brake signals, but all of this will be vehicle-specific.