Have you thought of one of these? They're like $20
Solar-Powered Battery Maintainer — 1.8 Watt
Installing a regulator/rectifier may be more trouble than it's worth - it seems the jury is still out on the capabilities of the magneto on these lil MBs... I've given some thought to the bicycle light friction generators as some are 12v, though there are so many disadvantages I've pretty much scrapped that line of thought.
There is quite a bit of relevant info on this in a corresponding thread if yer interested: http://motorbicycling.com/f11/12v-tu...ing-10176.html
Keep us posted on yer project though - I for one am curious as to what you may come up with