Time for some content: I am the aforementioned Spacerat working on the editor, and I bring you screenshots.
Currently the editor doesn't show you previews of the various entities, but at least they actually work!
Loading a new tileset in to the editor. It's also possible to do this automatically in a Lua config file - in fact, that's the preferred method of loading tilesets.
It's nowhere near finished of course, and frankly the core of the whole thing could probably do with a rewrite (writing GUI apps by hand is icky). But it's certainly functional and supports larger maps than anyone would ever need. The feature set is pretty slim at the moment; you can load tilesets and entities, place them, and save/load. Are there any more advanced editing features any potential map-makers feel the need for? In the spirit of this whole project, I might end up implementing them with some kind of Lua plugins system, alongside the Lua config stuff it already uses.
On the topic of GUI, I also wrote the game's in-game GUI module (windows/textboxes/etc). That's totally skinnable too, but I'm not sure if Catdaemon already has a skinner or plans to do it himself. Regardless that whole system will probably get a rewrite too since the current skinning system is horrible, and this time it may support clientside Lua scripting or even be entirely written in Lua. That's a while off, though.