Wow, you guys are awesome. Lots of stuff to try here. I think I like the one-pixel line thing best, because it would interfere the least with the look of the screen as I've planned it, but there's one crucial problem with that one: I'm already using two tilesets in that screen, and neither of them has a one-pixel line. So I'd either have to let one of the tilesets go and replace it with one that has a line like that, which would be painful - I'm a compulsive tileset combiner ;-) - or alter one of them to include the line... Hmm...
[Five days to go to my very last exam; after that, I'll be back at my proper computer, and also, officially jobless - so there should be plenty of time for level building... At least for a while. *g*]