@Invading Moss, thank you for the reply and solution! Unfortunately I can't respond to your answer as I chose this as a Question and not Discussion.
This does work for me too after my initial testing, but I need to apply it specifically to a single field. I think this will require me to attempt to duplicate the Drilldown dialog as it's shared amongst multiple fields (ie. LIBRARY.XML). Essentially, I want it to display PRIMARY system Locations for the ASSET.LOCATION field Open Drilldown selection and SECONDARY system Locations for the ASSET.FBR field Open drilldown selection.
I hope that makes sense. I'll try a few things while waiting for a response.