To be honest - the way I removed the back-wall is pretty crude.
It only replaces flickering tiles with empty tiles - which is kinda lazy...
Here is a approach you can try:
1. Download and apply the colorizing patch to a clean rom: http://www.romhacking.net/hacks/175/
2. Make a copy of the patched rom
3. Download the GB-colorizer tool: http://www.romhacking.net/utilities/30/
4. Open the rom-copy in GB-colorizer and JUST SAVE it.
5. Make a diff-patch of the Fixed version AND the, by GB-colorizer, messed up version - with LunarIPS.
If you/we are really lucky - this diff-patch will save the day!