Would still be happier about it if I hadn't had to use the
CONFIG_CFG80211_CERTIFICATION_ONUS sledgehammer to get to this point. Can't decide if it's more or less of a blunt instrument than the openwrt approach - https://github.com/openwrt/openwrt/blob/master/package/kernel/mac80211/patches/ath/402-ath_regd_optional.patch
This repository is a mirror of https://git.openwrt.org/openwrt/openwrt.git...GitHub
1100 words of English (or near equivalent) later and I've only just reached the point where the ath10k (used for 5GHz radio) is working
ath9k (2.4GHz) can wait for another day.