by just doing some quick maths i'm thinking that it would not really be worth the hassle,
if you are pulling say 50 amps then thats 600 watts, assuming that an alternator is ~50% efficient then you are loosing about 1.2kW, not counting pulley losses because they will still be there if you freewheel it.
if you have a motec or something that it is easier to do on then it would probably be worth doing.....
just my 2c