Sorry I'm late to the discussion... one solution that hasn't been mentioned is chaps.
There are perforated leather chaps for summer ridin' and full leather or lined chaps for other seasonal ridin'.
Ya see Harley riders in their pirate attire: bandana, vest, n' leather chaps.
Harley engines are air cooled as well.
I'm goin' on my 4th summer with burnin' calves, and I have lowers, so I commiserate with ya.
I've ruined a good pair of shark-skin Nocona's 'cuz of those pipes.
Presently, I'm toyin' with the idea of glass rappin' 'n silicone paintin' the headers (front pipes) to reduce the heat.