Damn, that's a good idea!Thats not a very 'elegant' system when it comes to engineering tho, its a bit of a bad solution.
The best solution with current technology I think is to ditch the traditional 'car stereo' completely. Use your ipod (which runs on 5v) as the interface, and have like, say 4 speakers, two left and two right, front and rear, powered by individual amps (we are talking amps in the electronic sense, not 'amps' as in 'big metal boxes with Fli written on them') running on 7v directly from the dynamo.
That way you get a combination of a modern interface in the shape of an ipod, but oldschool power amps which give you the best kind of sound, and can run from 7v.
Of course you cant just go into halfrauds and buy all this, it takes a bit of messing around, but a very good quality system can be made even on 6v cars.
I built one once and it worked pretty well. PM me for details if anybody is interested. The electronics is not that 'heavy' in it, its pretty doable even by somebody who is not so used to electronics.