I did two long rides (300K brevet & 100 mile invitational ride) in high heat last summer. What worked for me was to have a cloth tube filled with ice around my neck. I kept refilling it at every control/rest stop. I also would put ice in my sports bra (I had learned this trick when I did a 1/2 Ironman triathlon in high heat). While the ice never lasted until the next control, it did keep me from getting completely miserable. I am a snow bunny by nature so really do try to avoid long efforts in high heat.
Of course, I also drank lots of fluid and also used SaltStick capsules to help replenish what I sweated out. I'm a "salty" sweater.