You can directly edit the en-GB.mod_sclogin.ini files, but those may be overwritten as you upgrade.
The proper way to override any language strings is to use the admin area like you mention, which writes any overrides you create to the following file:
/languages/overrides/en-GB.override.ini
If you want to backup or FTP that file, you can easily do so to allow you to remotely edit.
That's the best way to override the strings of any extension.
I hope that helps,
Alex