The only ones D-V haven't tackled yet are those two and the O'Jays Live in London album, which was culled from the same December 1973 PIR tour of the UK and continental Europe that produced Billy Paul's Live in Europe album.
There aren't any beyond that, and I don't think there are any unreleased quad mixes either as far as I can discern. A couple of years ago I spoked to Arthur Stoppe, who did the quad mixes of four of the PIR quads (and assisted on all their quad mixing dating back to The Spinners self-titled LP) and questioned him about the subject, because Lou Rawls Unmistakably Lou is on the unreleased quad mixes list. He said that that title was definitely not mixed for quad at Sigma Sound, and in a more general sense he said it was unlikely that labels would go to the expense of mixing things for quad there and then shelving them because of the expense involved. Both the engineers and room itself were billed on an hourly basis, and Sigma was a busy, in-demand studio - when their quad equipped room ("Studio B") wasn't being used for quad mixing, it was being used for stereo mixing, so they couldn't give away quad freebies since they'd be losing money for hours they could be devoting to paid stereo work.
I know I'm grasping at straws, but my last remaining hope for The O'Jays "Backstabbers" was that it was an early or "big" enough title that it went to New York to be mixed by Larry Keyes. He was after all credited with mixing the first PIR title, "360 Degrees of Billy Paul". I'm hoping others may have been mixed as well. Hoping something in the "Pop" genre is in the pipe for August, too. Needing a good Quad fix asap!