Another reply to myself.
It’s easy enough to split out the ‘People’ data from the backup. The backup is just a tar archive, so you extract the file /People/data/all.vcf . This is just a text file consisting of concatenated vcf files, with avatar images base64-encoded in each card. I did this on the sd card.
Restoring from this file is simple if you have the File Browser App installed. You just navigate to the file and open it in the File Browser, and automagically, all the People/Contacts are imported, but it looks like they are not duplicated, which is helpful.
As it happened, I hacked out a bash script to split out all.vcf into individual vcf/vcd files so I could import individual details before I tried the approach of opening the concatenated all.vcf. I had thought I might need to extract the individual avatar images and restore them one at a time, but that (thankfully) turned out not to be necessary.
Looking at the People App, it would be interesting to understand why it doesn’t store the images in the SQLite database itself, rather than just pointers to the images stored elsewhere in the file system. I’m sure there’s probably a good reason.
Similarly, I miss the ‘Data’ app on my old Psion, as it allowed me to set up arbitrary flexible databases with a simple (text mode) user interface. Many people miss Apple’s Hypercard for similar reasons. (In fact, the whole Psion suite of software (Agenda, Data, Sheet, Word) was/is excellent)