How much weight can I put on a car rotisserie?

Aren't we just talking a couple of modified engine stands connected together? Get a couple thousand pounders at Harbor Freight. Lengthen the outriggers, grease the hubs and connect them with something. Since you're supported on the other end you can divide the torque applied when you have an engine hanging. Such as a 750 pound engine supported on one will multiply the torque over the length of the motor. I'm no PE by any stretch but 3000# is a pretty realistic if not light number on two 1000# stands.