As each branch should have a defined role and may or may not interact with other branches plastic scm needs a workflow management system (I'm inspired by Jira) which allows definitions of branch roles and let's call it "change/information flow" rules.

This would provide a way to define which branch can be merged into another branch and which can't!

Each branch then needs a role assigned at creation time. Even here the workflow rules could define which roles may be given to a branch created from another branch wich a given role.