I solved it.
The solution was (of course) obvious, once I realized it.
I had this:
master/: A copy of a broken repository, featuring revision 0..947, with revision 823’s files physically missing.repos/: A repository loaded from a backup (dump file), covering revision 0..910.
The solution was simply to dump from master/, from revision 911 and onwards. This was possible without any errors, which I take it means that none of the revisions in the range 911..947 directly depended on the state in revision 823, or something:
$ svnadmin dump --incremental -r 911:947 master/ > tail.txt
* Dumped revision 911.
* Dumped revision 912.
* Dumped revision 913.
[...]
* Dumped revision 947.
Anyway, then just apply the dump to the repository coming from the backup:
$ cat tail.txt | svnadmin load repos/
[lots of commits]
And now I have the full history restored, no problems:
$ svnadmin verify repos/
* Verified revision 0.
* Verified revision 1.
* Verified revision 2.
[...]
* Verified revision 945.
* Verified revision 946.
* Verified revision 947.
Yay!