Back when we were limited to one table per file, that was our only option and this is still possible in today's versions. If you open up the GTRR dialog, you'll see that there is an option for selecting a layout from the external table's file. Thus, you use a table occurrence of the external table in your current file to define the relationship, but then specify a layout in the external table's file.
Awesome Phil... Thanks, I never even noticed that option..