I've finally got an app that seems to be fairly bulletproof, I wrote it carefully to check on itself. There's no point having a backup if you aren't fairly sure that you've got everything. And all the metadata is backed up too.
While I'm waiting, I wrote a little readout page that shows the last ten pictures that have been archived, in reverse-chronologic order. It's kind of interesting to watch them roll by. Especially because it picks them in random order, so there's no telling where or when you're going to go next.