Replace one or both of these files:
\textures\interface\worldmap\wasteland_1024_no_map.dds
\textures\interface\worldmap\wasteland_2048_no_map.dds
There are other files in the unpacked texture archive. I'm not sure which ones are used for what or that all of them are used at any time or used at all.
Unpack the texture archive to see all the files. Replacing one or other or both of the above files should be sufficient to replace the displayed map.
I'm actually waiting for a colored version of the map although I don't know how useful it would be.
When I say "replace" the file I mean create a folder "\textures\interface\worldmap\" in the "data" folder and put a new file with the above name in that folder. Use the ArchiveInvalidationInvalidated! Mod to override the stock texture.