With ever more detailed reading of the XBee specifications, I've determined that we can't get the address of the ear tag without using the API mode of the XBee. Further reading leads me to believe that each XBee has to be pre-configured to use either the API mode or the direct mode. The default (from the factory) is the direct mode, and it seems the API mode can only be enabled by using Digi's xctu utility to upload the correct firmware to the XBee.
This adds additional complexity to the project and requires a MS-Windows system to run the xctu utility.
Note: I'll probably move development off the Beaglebone and to a more powerful desktop. Note2: I'm hoping using RTS/CTS won't be required for the XBee's - if so, it could interfere with the distance calculations.