Thank you for all the PMs 
Just a suggestion if you are game to do this yourself - arrange your uv islands so that the mirrored counterparts overlap each other so that what you paint on left will exactly mirror on the right.
Also, if you have enough geo to do it, set up flat material colors in material assignment and then bake the diffuse to your image, that will get you better edges. Also, you can use the face selection masking in texture paint mode by clicking teh little icon just to the right of the pivot center icon in the 3d view header.