That's a good question. I don't think I've had that issue since wearing a back pack in high school. You could probably get pretty creative with ideas to create space/air flow. I could see myself gluing strips of caulk backer to the vest, but that would create considerable bulk. Not to mention I don't know if that would effect performance.
I do remember reading about how doctors were using different methods of regulating people's body heat during surgery. Instead of using blankets to bring body temp up after heart surgery (where the person would be covered in ice) they would instead wrap warmers around their wrists. I believe the idea was that heating and cooling areas where major arteries travel closer to the skin works to effect body temp faster than just covering the whole body with blankets. So long story short, it may be worth looking into those cooling tubes you wear around your neck. Sorry I don't have any more valuable input for you.