Did some additional research. Looks like some military trucks came like this, with 12v and 24v terminals. I think it could be accomplished, especially with an external regulated alt.
One thing to keep in mind, with a higher voltage you are going to get a lot less amperage.(but same power, i.e. watts). I think to keep the rest of the car OK the high voltage side would need to be limited on amperage.
I'm thinking about 2-4amps per 12v battery. Not sure how this would be accomplished. I think a resistor would normally be used, but because the resistance of the batteries varies with charge IDK if it would work correctly.
I'm going to look into this when I have some more time. For future reference, here is a site with tons of alternator parts:
http://www.aspwholesale.com/index.php
