1) get default textures from the players.pk3
2) import the obj into blender
3) get a fully textured view of the model
3.1) open uv window
3.2) in edit mod, select the image for the surface/object(in uv window)
3.3) do the same for all surfaces/objects
3.4) go back to object mode
3.5) activate textured view
4) open the textures in gimp
4.1) paint over them in gimp
4.2) try to keep the shading
5) reload the textures in blender(in uv window)
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instead of using the default textures, you can paint over the baked ambient occlusion render
it will give a rough shading
that is what I did, not a good way but fast
edit:
some useless example with sources(gimp sources for skin, and ao parameters I used), dunno if it helps anyone
http://files.filefront.com/skelebotzip/;11991998;/fileinfo.html