I have just made a BG Map editor for SmileBASIC. It is very basic but very easy to use.
This edits maps of 128 by 127 tiles in size only.
The key is E5543NQE
Controls
Use the touch screen to draw tiles to the map.
Use the circle pad to scroll the map
Use DPad Up and Down to change tile. It defaults to tile 0.
press A or B buttons to switch tool.
0is a pencil, and
1makes boxes. X button allows you to import a tileset GRP so it's easy to see what kind of tile goes where. L button saves and R button opens files. Use standard
BGLOADcommand to open them in your own programs. There is a bug that I have found that breaks the program out of its main loop if a save is cancelled, so save even if you haven't finished so you don't lose your work.