THX for clarifying. First I need to learn what exactly the different K* values in a *.mtl file control, and how the according map_K* interact.

"External program" is the keyword for me. My final solution will probably be a script that reads the *.obj, finds and reads the *.mtl, takes the K* numbers of interest and accordingly modifies the texture file stated as map. With python or bash/awk plus Image Magick that should be feasible. I'm just too scatterbrained to do it manually with only few mistakes. Or maybe too stuck up.

It's going to be a case for the experience thread I just opened.