I don't know that I would consider it very time intensive making the rubble from sprues, as I have never considered it to be that difficult. I usually will clip the sprues up while I am sitting around watching TV and if you purchase a decent number of figures the plastic sprues you end up with produce a substantial amount of rubble. I could really not be bothered putting sprues through a meat grinder as doing it manually I can end up with about 4-5 cups of rubble while watching a half hour of television.
The plaster is a good idea too, maybe I will give that a try. I presume you must coat the plaster with some glue or something to keep the plaster from absorbing your paints and also to keep it from crumbling as it ages and is handled/stored.