Store a cell format parameter in a Text field and use it is a cube view

It would probably be easier to have an XFBR that simply accepts one parameter (the account name) and returns the necessary property. Then you should be able to use it like this on the CV properties: XFBR(MyRule,MyFunction,AccName=|MFAccount|). This would avoid having to mess with args, which might not be set in the way you seem to expect.

If it's about formatting, anyway, you can write a Cube View Extender rule; that has access to the cell, so you can retrieve the Account from there.