Ok I managed to figure it out after a bunch of googling. If you landed here via google and want to place a tile you cannot otherwise build do this:
1. launch the game in debug mode by adding -debug to the launch options via steam
2. login to the game as admin if playing MP and give yourself "brush tool" admin powers
3. right click in a tile and select "Brush tool manager"
4. click "choose tile" and scroll through the list of tiles to find what you want. The very top bar of the tiles menu that opens when you click "choose tile" is a search bar. Click on the tile you wish to place.
5. You can now place the tile in the game world by clicking on the tile you wish. You should see a ghost texture for the tile you are placing the same way you see a ghost for constructing normal walls, doors, etc. with the carpentry skill.
