Just for the reference: I fumbled with my MyBookLive's SSH settings, and was completely locked out from it. (I used both AllowGroups and AllowUsers in /etc/sshd_config; thought that it would work as a logical "OR", but it works like an "AND": the allowed users only have access if they're also from the allowed groups.)
I still had web UI access, but I simply don't use it — I'm under GNU/Linux, so I'm using ssh / sftp even for accessing shared files. I tried both types of factory restore, but it didn't help; I was starting to think that it'd be necessary to physically open it up (and change the messed-up file manually, or, if that didn't work, follow this procedure for completely bricked units).
Eventually, I found out that a firmware upgrade did the trick. I downloaded the current firmware from WDC's website (the same one I had installed!), and updated it from file through the web UI; upon reboot, SSH settings were back to default.
I'm kind of glad that I had not disabled the web UI. If I had disabled it, I would have no option but opening the unit and completely debricking. (I know it can be done, but I'm *sure* I'll break all those clips. :-)