I've been riding, and working in bike shops my whole life, and I've never seen the seat of a cruiser get as hot as your suggesting. Some sport bikes that run the exhaust under the seat are the only excepion that i can think of.
That beeing said, I have a hand made solo seat that was given me by a friend that has Kidex as a seat pan.
Not trying to be argumetative, just stating what i know from experiance.