> Generator attached to the flywheel instead of the starter motor, and then find some way to squeeze a drive motor by the rear diff (probably more complicated, but surely not impossible).

Some vehicles have this as a “battery starter generator” where the “hybrid” functionality is entirely on the accessory belt.

https://www.bosch.com/stories/48v-hybrid-battery/

https://hackaday.com/2025/01/24/bosch-starter-motor-freed-fr...