Business Roles concept and usability in GRC AC10 - Governance, Risk and Compliance - SCN Wiki
the link above says that "update assignment" button will do the update and will be enabled when the business role has been provisioned at least once.
I guess this is what you have already tried, but i can see your dilemma when you may have many business roles. I wish there was an option under the mass update functionality (unless I have not found it).
maybe it's time to go to #ideaplace.