"It may be possible to just rectify the ac, and charge a large capacitor, and to run the existing DC/DC as the charger"
I see no reason that wouldn't work, and I would be comfortable with that technique. You would definately reduce weight, simplify the electronics, gain space, and perhaps even gain a couple of MPGs on a really long road trip. You would also castrate perfomance and trash the city MPGs.
Personally I'd rather see a really big IMA pack, perhaps using one of the emerging "safe" lithium technologies. In a few more years such packs may be available from scrapped Prii, which by then might be selling a million a yearl!
