I've thought of that before since I don't really have a use for loam, except it was with tree patches instead of plots. But I agree with Millie about the whole interconnection thing, and it'd be a good use for those of us who aren't that into street projects.
I dont see why it should bother blockmakers per se; whatever loam may cost just get passed on to the consumer anyway. It might slow down street projects slightly, which is probably great, but the real cost of blockmaking is how. Goddamn. Slow. The blockmaker is. The real flaw in this otherwise cool idea is that this would further elevate the cost of gardening, and i really cant see the advantage to that. maybe once the produce vendors are all shot...