Smith Koroyd or Bontrager WaveCel. They seem like they'd be hot and heavy to me, but several friends wear them in Texas summer daytime rides and say they won't wear anything else. The liners would seem to offer more sun protection.
I prefer regular Bell and POC helmets, but I have long hair and often wear bandannas. Depends on the heat and humidity. I prefer no bandanna when it's hot and dry because the sweat evaporates quickly. When it's humid the bandanna helps absorb sweat.