I thought this might be an issue, too, but I assumed there was some decision made that I didn't know about. I can attest to the fact that, in the original, it WAS possible to get down to zero weight. The only things turned on in UFOExtender are inventory stats, D3D, Cursor Clipping, and Cursor Scaling. ALL bugfixes are turned off (as in not fixed).
I unloaded the rifle, then dropped everything down to the ground.