Sometimes I hike with people for whom bare chest or even bare legs would be uncomfortable. But I get overheated MUCH more easily than most people; anything about 70 degrees is too to wear clothes while hiking.
I'm thinking that it might work to wear some super-skin-tight fabric that is so porous that it doesn't add any noticeable insulation or moisture. What fabric would that be? Rayon? I wasn't able to find any rayon-based baselayers in a google search.