This is actually a quite snazy idea! I can only think of one problem: Noobs. If a newb was hosting and had this and wanted to use it, but didn't have f11 or common sence, they would ask alll sorts of dumb questions on how to get it working, and end up never getting it to work.
I think the best bet would be, to make it a wand-type-deal. Have it so you hit certain plates/bricks with a wand that makes them the only things you can build on. To place the plates/bricks would be super admin/admin only.
But, I do like the idea, I want it! (good for rpg's/cities)