Allow Business Unit as a refinement criteria when referencing marketing emails in lists

Currently all marketing emails have to be assigned to a business unit yet there isnt the ability to use this value as a refine,ent when referencing emails in Lists.
We have 6 business units and would like to created unengaged lists per business unit. Currently there is not scalable way to do this yet the value must be stored as it is viewable in the email overview.