I think Hendrich here has the right idea, actually. It would make more sense as part of the layout system, since it is a modification of buildables on a map, as opposed to its currently implementation as an admin command.
Picture this more: In devmap, there's an extra nobuild-marker buildable (available to both teams). When you "build" it, the shadow buildable then becomes a second one that defines the other corner of the bounding box. A third one could then come up, which could either be placed or cancelled, giving the region a height (or not). The markers and affected area should be given some reasonable graphics (visible only in devmap of course). They should then be included in layoutsave, and loaded (minus the visuals) in layout load.
I understand that the graphics-side of things isn't really possible in game.qvm only, but I personally don't think it'd be a bad addition to the mainline layout system in theory if done cleanly along those lines.