If 150F is melting a seat cushion...maybe this is one of those "Who'd a thought?!" tales where the real problem is that the owner just recently had that seat re-upholstered. The wrong way.
This owner does very little. So no chance of any stealth seat cushion work being done.
I did notice that the pressure cap with a vent is right under the seat cover. I'm wondering it it let out a little steam?