Hi Travis,
Your question is clear now. I've never worked with BEx query as the data source, Sorry, I couldn't help you further on this this. I'll let Gurus respond to this. But, I hope this is a way to achieve what you want, I checked the list of functions available (in BO 4.1 SP6) and I see a Descendant(), hope that function will help.
Thanks,
Mahboob Mohammed