Partly answering my own question after mulling it over, the difference in the pre-pass is that you're not actually coming "from" an edge or going "to" one when you're doing the flood fills, you're just testing which edges are connected to any given empty cell. But I still think there might be something to this idea. The simplest thing I can think of is to do no-backwards searches out from all the empty cells on each edge and see which other edges you reach. That's potentially 6x as many searches (slightly smaller and with some early outs since the connectivity is bi-directional), but at least it's still finite and predictable and isn't who knows how many raycasts. I don't know if that's acceptable for the pre-pass or not. But I also think there might be smarter ways to do it that only require a single search from each empty cell but remember directions traversed so you know if a particular path you've reached an edge along implies visibility or not.
Maybe you can do the 6 searches in parallel, such that whenever they meet you know you've found a connection between the two edges they were coming from? I don't know if that means any less work except in cases where all 6 edges are trivially connected.
Maybe you can do the 6 searches in parallel, such that whenever they meet you know you've found a connection between the two edges they were coming from? I don't know if that means any less work except in cases where all 6 edges are trivially connected.