The coolers rock, and they do NOT make you hotter. I was suspicious about that last summer but bought a set of arm coolers - it did help. I am in the Midwest and it might be different in a hotter/drier climate.

It looks like the heat rash I can get from my PI grippers - back when I could actually ride for longer than an hour... but you said this was exposed skin so it isn't the same thing. Have you tried squirting a bit of water on your thighs from time to time?