Car alternators are 3 phase DC excited synchronous electric machines. They can be used as a motor by a proper 3 phase motor driver.
Let's think that we connected a 200Ah Lithium based (eg. LiFePO4) reservoir battery parallel to car's default 12V battery by a step down converter. As we are driving, step down converter is turned off, so our reservoir battery is disabled. When we need to apply brake, we simply press the brake pedal which now has a pressure sensor on it, seamlessly controls the step-down converter's output current (which is roughly equal to the output power, thereby the braking power), which in turns slows down the car without the need for the actual brakes.
The maximum breaking power with a 200Ah LFE battery will be roughly around 2kW according to my experiments with a 60Ah LFE battery connected parallel to my car's battery.
When we are driving normally, the motor driver connected to the generator slowly starts driving the generator in motor mode, which in turn make us feel like we are starting to drive downhill so we simply release the gas pedal accordingly. The motor driver will use the energy of reservoir battery as much as possible in order to open as much as space for the next braking event.
In this scenario, I guess the mechanic strength and aptitude of the car alternator that is required for the maximum allowable boost power in motor mode is equal to the maximum continuous output power of the alternator in generator mode. If the alternator is built to supply 100A continuously, then it generates 1380W continuously, so all construction would be appropriate to use it as a 1380W motor.
Is that accurate or can't we use the alternator in motor mode for some other reasons?