I agree, though I am curiously waiting to see whether there is a way to achieve this, I do not believe there is. If there is nothing returned from the rule, the assigned target value from the Composite rule should be returned.
If you do not want to put your list mapping into the Composite complex expression (which is a valid hesitation!), one could also use lookup tables to refer to and store the list mapping used in the complex expression (and keep the lookup table in a global variable).
List mappings with a complex expression should work as well as per DanielWillis suggestion.
If your only purpose to apply those two case mappings to your BS accounts, Mask mapping with a complex expression or business rule might be another way to go about it. Up to you to determine which solution suits the requirements best.
Yes, definitely also use a global variable rather than looking up the list each time.
