So, having failed to make any progress with the travel guitar I had a go at the RS8V.
I got the nut off the guitar and compared it with another 80s Washburn locking nut kicking I had around which appeared to be the same. It was but 42mm wide rather than 43mm.
While not being quite the right width it was available and free so I carefully attacked it with a file taking a little off the base while trying to keep it nice and flat. I figured that locking nuts were cheap and easily replaceable if I screwed up while the neck of the guitar was kind of harder to replace.
Having taken about 0.2mm off and managed to keep it flat enough, it's not a cylinder head after all, I went to test fit it to the guitar. Half way in the first screw snapped, it must have been made of mystery monkey metal too I was hardly applying any pressure.
So I had a little go with some needle nose pliers trying to grasp the snapped off end, but had to resort to cutting a small 'moat' round the stump with my Dremel to get any purchase.
This wasn't too productive an evening, I'll quit while I'm behind.