OK, on another forum (warboards), some modder made a dweb that you could not cast spells under, and an AoE restore (that cannibalized maelstrom).

That person is now gone, and I don't have the mod files (must have lost it when I reinstalled Windows).

Anyone know how it could have been done?

I vaguely recall seeing this too. IIRC, it was via plugin/scripting.