A while back I blogged about the wide variety of clips available for stretching fabric. Most of these are sold as tarp clips, but Rosebrand does market versions to use for for theatrical use. Today I ran across another type used for netting: poly clips.
The link above takes you to a PDF about them. I wonder if the teeth would effectively grip webbing without damaging fabric. I liked that they incorporate a wire guide, but I can also see this as being ineffective- the way the pull is orientated it could be difficult to get enough stretch - you would have to be able to tension your cable enough.