Trying to figure out a way to set the og audience of cases to the same audience of related project

Using OG I make projects only visible to parties involved. Parties consist of several users grouped with OG. On projects I set the audience to non-public plus each OG of an involved party. This way I can manage the parties/OG and give users access to projects dynamically.

However when users add a case I would like to be able to confine the case to the same audience as the project related to the case.

An alternative solution would be to create an OG per project, limit the project audience to that one group and just add users to that OG. Problem with this solution is dat new users belonging to a party/group do not automatically get access to all previous projects/cases.

I have also tried playing around with workflow and actions to automatically add new case nodes to a certain audience. This would perhaps be a solution if an action would exist to set the audience to the related project audience...

It there a way to accomplish my goal (other than writing php code)?