I was searching for symmetry texture painting and found you guys post. As @Sanctuary said symmetry texture painting isnt there (yet). and I would also use gimp as primary texture editor along with texture painting in blender. So I played around with gimp a little bit and found some little tricks I want to share.
Using mirror modifier it will always symmetry, when i want to make a mole only in left it will appear in both left and right. Even after I applied. (dont know what i did wrong ^^ )
in Blender after unwrap, I see an option called: copy mirrored UV Coords. it will make uv symmetry.
then we can edit that picture in gimp (only a half) then flip that half and we should have symmetry.
Problem I found that I dont know how to select exactly half of picture to flip in Gimp
To solve that problem I used View/Show Grid and Snap to Grid
then Image/Configure Grid. for example I used 1024 x 1024 picture, so I changed spacing width to 512 (half of 1024)