I think you are overestimating unique requirements of typical car engines. They are usually DC powered AC engines, where the DC->AC converter (generating 3-phase AC of controlled power and frequency) can probably run backwards (AC->DC) with at most a few minimal hardware changes, if any.
If you're not overdoing regen, you probably don't need additional balancing. Even if you wanted to charge the EV by towing, you could probably use the normal charge balancing circuitry, again minimal if any HW changes. Non-wimpy batteries and cells should be fine - if they can fast-charge, they can take regen. Might have some limitations on acceptable power vs. temperature, charge state etc.
If you're not overdoing regen, you probably don't need additional balancing. Even if you wanted to charge the EV by towing, you could probably use the normal charge balancing circuitry, again minimal if any HW changes. Non-wimpy batteries and cells should be fine - if they can fast-charge, they can take regen. Might have some limitations on acceptable power vs. temperature, charge state etc.