i've thought about this, but only reusing the negative plates (turning half of them into positive plates) after the battery has already reached end of life.
basically the plates are optimized for the unique chemistry each faces, and positive plates will hardly survive being turned into negative plates due to mechanical strength issues and the resulting shrinkage (afaik, the active material will shrink and detach from the grids) negative grids seem to handle it just fine from what little testing i've done, but i'd imagine that they would have nearly zero capacity as the pore sizes would be all wrong. i wasn't able to get any significant capacity from them, but i didn't cycle them, just reverse charged a few negative plates to see what would happen.