This involves four relatively minor changes.

1) adding scheme (ftp , http etc) and port information to AdFileSystemDatabaseBackend init and add database panel.

2) replacing any unarchiveObjectWithFile: with a) dataWithContentsOfURL: and unarchiveObjectWithData:

3) Updating checking of directory structure - can't use NSFileManager with non file: scheme urls.

4) Addition and storage of passwords.

The only problem is that currently OS X only supports reading from ftp and http urls. You won't be able to add data to databases on remote ftp & http server.
GNUstep actually has better features then cocoa in this department as its possible to write data to an ftp site through it.

Note: URLS cant handle spaces in path names - have to be aware of this.

Users then add databases using URL syntax

scheme://password:user@host:port/$PATH_TO_DATABASE.

various elements of the "password:user@host:port" are optional depending on the scheme (e.g. file:// doesnt require any of these things).