I don't think there's a way to do it completely flat, but if you're talking about molle/pals webbing loops, you can just make them a little wide, a little slack and turn the 1 1/2" normal length into maybe 2" and angled, to effect the curve. If you're talking flat webbing, without slack, I don't think you'll do it perfectly cleanly on a curve. I can't think of where I may have seen it done as a sample to copy, either.