The post arrived, while I was busy on the dome, bringing a 1.25" 90° T-S star diagonal from a UK dealer. It was much better quality than my no-name examples from several years back. Most importantly, it had a compression band eyepiece holder and a decent sized thumbscrew to get a firm grip on the binoviewer nose-piece. The diagonal is delivered in a posh, textured cardboard, presentation box. With a cut-out foam box filler for protection.
I briefly tried the binoviewer in the star diagonal and, as expected, it was securely held by the diagonal's compression band.
However, once the GPS was fitted it proved physically impossible to clamp the nose-piece of the binoviewer itself, even at full depth. I had to lift the binoviewer just enough to completely clear the compression band. The diagonal then held the GPC directly. Being of sturdy aluminium build, the GPC can be safely held by a compression band. Though it does require that the GPC is screwed firmly to the binoviewer to avoid any unwanted movement.
The technique is to tighten the diagonal's compression band just enough to prevent the slightly larger, binoviewer nose-piece from entering. This automatically stops the binoviewer from dropping down too far and leaves the GPC at the correct depth for secure clamping. The lower, right image shows the binoviewer nose-piece completely exposed after GPC clamping.
The T-S diagonal's eyepiece holding section is easily unscrewed. So, the binoviewer nose-piece is being held by a very similar sized screw thread to that of the diagonal. The GPC + binoviewer is just one more thread in a whole series, literally holding everything together. Any weakness in the system is [probably] largely psychological. Provided, of course, everything is screwed on firmly, but only hand tight. Tools are not desirable for this task.
I must admit that I still find the unwanted and undersized GPC extension a design flaw. I would much prefer the heavy binoviewer, with its two low power eyepieces, be properly supported by its own nose-piece. Even if only partially. Avoidance of unwanted extension is provided if one simply leaves the GPC off.
Which is exactly my plan as I build a terrestrial 90mm binoviewer telescope for typical spotting scope powers. Rather than, more typical, astronomical powers.
Where full, binoviewer nose-piece seating, with GPC, is desirable then a 2" star diagonal can be used with a 2" to 1.25" adapter. Though here clearance for the physically sensitive mirror must be carefully checked. Or the binoviewer + GPC can be used "straight through" in a 1.25" focuser without a stop ring.