the inability to link between multiple files has been a complaint almost from the day we got the ability to link.
The closest I've seen is to completely split the building into one file and the layouts into their own file. It's a little clunky, but it does allow you to always link to something in the same PLP file while removing the huge amount of redundant data in the layout book.
I like what you're trying to do, but GDL isn't smart enough to know all the data you would need to export an object's location to an external file.
Tom Waltz