My Alien Dalvik was behaving weirdly, so I deleted vending.apk and vending-1.apk manually, then installed play store 6.9 via MTP and a file manager, which appeared in /opt/alien/data/app/ as vending-2. So I touched vending, vending-1 and vending-3 (for good measure/luck) and made all four chattr +i (immutable). And it is all working and hasn't auto-updated so far, even when I poke the version number to manually self-update.
I have noticed that the All Apps tab adjacent to the updates screen doesn't work any more though, but no great loss.

Many thanks Ancelad + Bundyo!

I have a feeling some other 7.x version will work too, though my original 7.6 did work a couple of weeks ago, but manually installing 7.6 now doesn't work (maybe Google are doing some a/b testing or something). I'm happy to stick with the latest 6.x, even though the update UI in 7.x is a bit nicer.