Hi Eric!
I am having a similar issue as the OP and I've been doing some testing to try to get this to work. I have an XFBR that attempts to use a passed account value to return that account's UD2 constraint. That feels like a good candidate for using the |MFAccount| substitution variable in my name value pair for my XFBR. I've tried using it in both Member Expansion 1 (i.e. A#Account:U2#XFBR(...,SelectedAccount=|MFAccount|)) and Member Expansion 2 (Member Expansion 1: A#Account, Member Expansion 2: U2#XFBR(...,SelectedAccount=|MFAccount|)). Neither of these appear to be working. However, this works perfectly if I am attempting to use for Name to rename the row (i.e. A#Account:Name(XFBR(...,SelectedAccount=|MFAccount|)). That helps me know that my XFBR is working. It also works if I hard code my account (i.e. A#Account:U2#XFBR(...,SelectedAccount=Account)).
Are you aware of a way to use |MF| substitution variables when they are all on the same member expansion level to generate a dimension selection or can it only be used in an XFBR when generating a name?