In an exchange with Scott on this board he said he uses an Oxy 30lb wing (IIRC) with a TP by attaching it with a single tank adaptor. I'd never thought of doing this with a single tank & my first thought was that the bolts would surely dig into the back. He assured me that this is what he had always done & it worked just fine. I never did try it myself, but if it is the case almost any wing with grommets at 11" centres could be made to work.
Yes, the Oxycheq Mach V wing can be used with the Transpac. As far as I can tell, however, one must use a single tank adaptor in order to attach the cam bands properly. Cam bands can not be threaded under the center line of the Transpac. Instead, the Transpac is made so that the cam bands get threaded under the 2" webbing that runs to the right and left of the center line - which is covered by the Mach V wing. (The Mach V wing does have cam band slots, counter to some info on the web.)
I use Oxycheq's regular "lightweight" STA (ROWB-06-02). It comes with stainless steel mounting bolts + wing nuts. When mounted as indicated, the wing nuts look like they'd stick into your back - but, in practice, once the Transpac's lumbar pad is in place, I couldn't actually feel the wing nuts at all, even with a full tank and no wet suit. That said, I use DiveRite sex bolts instead, partly because they're flat on both sides, but mostly be because they're actually longer - i.e., more threads are engaged - than Oxycheq's mounting bolts.
I experimented with Oxycheq's mini STA (ROWB-06-04). But I was concerned that these can rotate, e.g., in transit, and that the mounting bolts could loosen. The one-piece STA can't do that, because it's attached in two places.