You're right that the aliases won't work, but you probably read about using the LW Library file menu to choose "Add Folder to Share" to select the folder on your external disk. Didn't this work?

The new files will take a fair bit of time to load (getting 'hashed') the first time (or if the the drive was disconnected when LW starts up).

Cheers --sorry if this isn't what you were asking.