Well, my wife hates me using Amazon, but I ordered the part last night and it arrived a few minutes ago.
Success! Plug'n'play. An assortment of resolutions to choose from and the 2560x1440 @ 60Hz option pretty much fills my iMac 5K screen. Even better, a square is actually square on the emulated screen.
Although I purchased the Affinity replacements for Adobe CS to run on the new iMac, I had to fall back on Photoshop on the Mac Pro the other day – the small screen size was a bit of a problem, but not any more.
As an aside, I was doing without sound from the Mac Pro, but have now connected some bluetooth ear buds to it and it's like having a whole new computer. Nice.
Anyway, thanks for the ideas.
DVI Dummy Plug In case anybody's interested.
