CDR's link above might be onto something. Otherwise not much else practical. The Nike stuff for basketball might be light enough to breath but does it go on under your shorts or over? Maybe if someone made something like that with a chamois? Also that stuff is made to absorb an occasional knee and to slide on a wood court. I suspect asphalt would tear it up. Crashing already is expensive.

Knees, forearms, elbows are particularly rash prone and I'm not sure what would cover those and still breath.