Ok, partially answering my own question. It turns out that the kernel image had been corrupted so I had to restore that as well.
The “Slot A” kernel was in /dev/sda2, “Slot B” kernel in /dev/sda4 - “Slot A” root filesystem in /dev/sda3, “Slot B” in /dev/sda5.
dd if=/dev/sda4 of=/dev/sda2
dd if=/dev/sda5 of=/dev/sda3
HAOS can now boot just in from Slot A - HOWEVER “os info” tells me Slot A “version: null”. I would still like to fix that!
Looking around in /mnt/boot/EFI/BOOT I can see where the EFI boot config is all setup, but I don’t see anything there about status, version, etc - so still not sure where this “boot slot” metadata is being stored…