OK, did some more exploration; we're in luck
- at least partially: it turn's out that you don't have to
replace the colorconv_sepia.tga file - the game uses the first it finds, so if you have your own version, and the file has the same name, but it's in a different folder, it's enough to (don't do this though) add the path to that folder to the resources.cfg file
before everytheng else (more specifically, before the "/textures" folder).
Now, since full conversions can be configured to use
their own resources.cfg file, a full conversion
can use a different color replacement map. I was hoping that something similar can be done with the shader itself, but I don't think so, since it's in the "core" folder.
Although you can't get a grayscale image using the color replacement map alone, you can nevertheless get some pretty interesting effects; check these out:
(
large)