If you have a problem with the pipe warping at all, we were thinking about "quick-fix", also called "fix-all". It's a powder you mix with water for repairing walls. When it cures (starts getting too hard to work with in a few minutes) it becomes tempered like porcelain. Would add a little weight, but not a lot and should keep the pipe from warping. This would be a bigger problem it hot areas, plastic pipe don't like heat or sun. Unprotected PVC turns black, warps and twist in the hot sun like you took a torch to it. You would need to seal the pipe though after it has fully cured because getting it wet after wards causes the stuff to break down.
When mixing, add enough water to make it creamy and it will flow nice.