Zippers are by design very flexible when they are opened. There is space between the teeth and the coils so they can bend quite significantly. I've run a zipper around a 90* arc and still had it zip up and open with no problems.
The issue of weight is really a non-issue in my mind as the flexibility of the zipper compensates for those forces. One's butt is very adept at spreading weight load out over space resulting in low per inch pressure areas. I would not worry about it.