I'm not familiar with how much power the window AC units use, but wouldn't a DC to AC inverter work to power it? It would reduce the redneck factor by at least half though....
Some assumptions have to be made, but it should be pretty likely to work fine.
Car alternators can output at least 50 amps generally (700watts). Small (5000btu) Window air conditioners uses about 450w, even if the conversion is horrible, it'd still be at least 80% efficient, so 560 watts would be available for the window air conditioner to run from.