I bet it a sprite that automatically is removed when it's duration is up. I'll have to experiment with some of that, but for now I might have the solution.
If I use the add on construction site, and I use a bunch of them, they are all overwritten. It works out great. Almost like you are stepping through the rubble. Now I need to see if I can use this to make some walkable water too.
The only problem I have now is that I dont want this to show up until someone blows it up. Buildings are pretty much the only thing I'd be able to use, and they just go right below it. As I am writing this, however, I think about air units...
So what air units might not move much or look weird? Or what about a building that could fly. Oh wait.
And a few seconds later...
I'm still looking for some more ideas, though.