it's easier if you use an analog cable as the "redundant" connection and not a digital cable, then you don't need to assign anything (just make sure the "name" of the RCA input that you plug into matches what you are using for the input) and you are guaranteed a "compatible" signal (since you can only take 2.0 PCM to Zone 2 if it's digital).
You don't actually want to assign a different name -- the idea is that, whether you select your "BDP" source in Zone 1, 2, or 3, it will play the BDP!
So, for example, if you have your BDP using the "DVD" name, just run standard RCA interconnects (red/white analog) to the "DVD" inputs and you will have a redundant analog audio feed that is matched up to the digital feed for Zone 1. Then, regardless of which zone you select the "BDP" input, you will get audio...
Have you read this? http://batpigworld.com/fadq.html#multizone