Looking back at MetroMPG's page on alternatorless living,
Plug-in Blackfly: going alternator optional nets +10% mpg - MetroMPG.com
I saw that the car cited, a 94 civic, needed about 165W to run the engine and fuel pump only.
Maybe 3 of these in parallel, with beefed-up connectors?
Still looking at NiHAO Mike's suggestion...