The power supply is the easy part. I have 3 different RC car battery chargers, and I think all are current controlled. Power supplies will likely be the best option, although they have low current maxes usually, so I'd measure the draw of the ECU while flashing before I'd try one of those.
Alternators have voltage regulators on them that bleed off excess voltage from the alternator, I wonder how useful that could be coupled with say an 18V power supply.
When I have free time I'll dig through this, electronic's of this sort are fun.