Comparing to what they did for SH3 Navimod, do you want to upload what you have so we can all take a look? More people working on it the better.
I do not have the NewMap.tga though, since my Graphics skills are below par.
But I can try to figure out what the differences are and maybe get a work around.
I need to know exactly what you did to reproduce the problem, it may not be possible to do it just like SH3 but that old command left over (tool helper icon -RetroTool Mod) from SH4 can be tied to pop back and forth between two graphics, just replace the one tga with this new overlay (position etc) and with one click you can flip it on and off. I guess. Upload what you got and I can give it a try, or you can use that idea.