I know it's likely to change drastically some time, but for now I wonder if a few simple changes could make things much faster.

1. When selecting a piece from the inventory, Tab should go back to the next higher level category (or exit if you're at the topmost category, like it does now).
   
2. A couple of keys to cycle over items in a category without having to use the inventory would make things much faster, so you can, say, switch between characters rapidly.
   
3. A copy current piece key, which changes the "brush" to the piece on the map the brush is over (eg: if you're placing walls and you move the brush over a Porter Exit and press [COPY] you are now placing Porter Exits).

IMHO these could greatly speed up editing and would hopefully take almost no time to code.