Hi Scott,
I've got this to work for me.
1. Mount whatever volume you wish backed up.
2. Create your New Backup request in the Backup app.
3. Choose the kind of Backup where you select the files/folders.
4. In the Browser for selecting, just keep navigating until you hear
some combination of numbers that really make no sense. Stop
interacting with the Browser and navigate to the right so VO will
tell you what is actually selected. It does allow you to select
mounted volumes. Once you have figured out which one you are on,
then tell it to "Include this folder".
5. Continue on in the normal way you would to choose location and such.
It should work for you although it's not pretty.
Later...
Tim Kilburn
& Carter the Canine
Fort McMurray, AB Canada