PB631, FW 5.2, works. Good catch!
A couple of notes:
- If you ssh as root using ezdiy's jailbreak, you don't need sudo, because you are already.
- I initially thought of just copying device.cfg inside /mnt/ext1/system/config/, because several other default settings are overridden by files in userspace, but it didn't work.
- As a matter of fact, the opds urls are saved in the file /mnt/ext1/system/config/opds_catalogs. The file can be edited externally, but the changes appear to be reflected in the Library menu only after reboot.