In reply to kritter:
I am reminded of an anchor used by (some) canyoneers... a tarp, the four corners tied-in and bunched together, filled with rocks and gravel and sand, and a tagline tied to the center of the tarp. Once you've made the rappel on a single strand (anchored to the four corners), you tug on the tag and the gravel is shaken off the tarp, which you can then pull down.