Larry,
It's been in the high 90's for month now in Atlanta.
I've been putting sheepskins on the front seats of my cars for years. They're confortable, they make the seat easy to slide into and out of, they are cool in summer and warm in winter, and they protect the leather seat.
I've been using Avril Exclusives in Atlanta as a source of these sheetskins, and I've been impressed with the fit and quality. I'm using them now on the comfort seats of my '97 540ia. When I get my M5, which will have black seats, Avril will be one of my first stops.
The extendable thigh support of the M5 seats might prove tricky, but I'm sure that Avril has a work-around. For instance on my 540ia, the sheepskin passes beneath the adjustable head support, so that the support retains full range of movement.
You should give Avril a call (404-266-3678).
Richard