I did some further investigation and this was when I discovered the “Host Protected Area” (HPA) feature had been activated. At a guess, either the kernel or the disk itself activated this feature to prevent further damage to the troubled areas, but if you don’t know about it and don’t do deep enough research to find out about it, it could be easy to assume that the data is unrecoverable.

As the wikipedia article linked above mentions, a bit of hdparm magic (in my case thanks to sysrescuecd) and your entire disk will be visible again, leaving you free to attempt some data recovery (I used ddrescue and the errsize was only a few hundred K – haven’t found which files yet, so it’s likely data I don’t care about or never use!)