Can Landfill be limited to shallow water?

[grimdanfango]

It's simple enough to disable Landfill outright - which I'm leaning towards in a mod I'm fiddling with, as for my planned purposes, it makes for a thoroughly overpowered tool.

What I'd love to be able to do though, is to limit it to only allow placement on shallow water squares, so it's still a useful tool for occasionally linking up across shallow archipelagos, but can't be used to cross entire seas. It wouldn't feel nearly as much like a cheat to me then.

I'm assuming this is impossible though, given the fact that creating new land automatically creates a 1-square buffer of shallow tiles around it, any way to limit placement would be negated by the fact that the player could just just expand it a tile at a time.

Can anyone think of any way this could actually be done? Maybe some way to detect and enforce that the placement is only allowed when there's at least a 1-square buffer of shallows around the cursor, so it can't create shallows where they don't already exist?