Sorry I haven't looked around at too many other people's tilesets, but I am assuming something like this has been done before? If not, are we sure the toolset can handle it?
I know for a fact it can be done with my method of tweaking the GFF tile indices in the area file, and after you do that, it will load in the tileset just fine (might not be able to alter it though without a crash).
Is there an example tileset that actually accomplished this with the toolset? I'd like to see one, or the screenshot of that tile in use.