So, I had a brief look after getting kernel 4.18.5 booting… sure enough the problem was I had forgotten the watchdog, although I did see btrfs trigger a deadlock warning, so I may not be out of the woods yet. I’ve posted the relevant kernel output to the linux-btrfs list.
If I try to add a second driver, they’ll clash, and I expect the same if I try to access it via userspace. So the sensible thing to do here, is to add a ts-companion.c MFD driver here, then re-factor ts-wdt.c to use it. From there, I can write a ts-psu.c module which will go right here.
I think I’ll definitely be digging into those older sources to remind myself how that all worked.