How about this: When I was still working at the massageschool, my fellow training victim, Bob and I tried to find out how many of the students we could hold at our stomachs. Bob was large enough to have 8 girls standing directly on his stomach, but on my stomach there was only room for six. Problem was indeed that the weight was spread out. We tried the wooden brett you mentioned, but Bob was so strong that we finalley had a brett that was about 1.5 ft wide, 3" thick and 8 ft long. Believe it or not, Bob held 11 women on that brett. The problem we had was to keep the brett in balance with the girls on it. Eventually Bob turned out to be stronger than the brett because it broke just in the middle, above his stomach. Still crazy about the idea about the maximum weight we could hold, my inside architect and constructiondeveloper began to think. We constructed a base platform with on all four sides a standing steel bar. We took an old closet door, attached a metal eye on each corner, so that the eyes would slide over the steel bars. If you have trouble to picture it: How about an elevator cage in its shaft. The closet door would be the elevator floor that could be lifted from the platform. We could lie down between the door and the base platform and the girls could easily step on the door without the door tilting to one side. At first we forgot that starting fromthe third girl the pressure would be spread over our chests and midsection so we took an old fashiondoll (the full size dummy mannequins used in fashionstores), removed the feet and attached those to the closet door. Finally we could lie down, position the dummy feet on the right spot on our stomachs and let the girls step up. For your information: Bob managed to hold 9 girls with a total weight of 1070 lbs for about 3 minutes. That includes stepping up and down again. About 1.5 minute for the complete weight. I stopped at 6 girls with a total weight of 740 lbs. The girls weighed between 110 and 137lbs.
Bob was an ex heavyweight wrestler and weight 320lbs; My weight was about 200 lbs.
Just keep in mind: using a free-floating piece of wood is not practical to support more than five girls: one directly on top of your stomach and two on either side. More girls will make it difficult to keep them in balance.
Good luck, Massagerabbit