With many people wanting a painting app that does realtime tiling and mirroring and be pretty much like Photoshop, Viewport Canvas seems to me like the perfect solution. It has come a long way and while it might not be perfect for painting on volumetric models it does everything i could imagine someone would want for handpainting tiling textures. Just apply it to a moderately subdivided plane (too few or too many subdivisions can be bad for performance) that has it's UVs tweaked so you actually paint across the seams and have fun.
Just figured I would share this in case anyone wants to mess around with it.
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It might even flow nicely into another idea someone else had about creating tile textures for boxes/spheres...
http://www.polycount.com/forum/showthread.php?t=102021