you can achieve it by user status so when project object is in Allow to PR condition then system will allow to create PR.
opposite it if project object having the status Not allow to PR then system will not to allow the creating PR.
just check business object of PR as RMBA is the status of that allow or any other ? once it would be allow then system will allow to create the PR for that object which have status allow, let me know if any thing else.
Regards,
Sanjeev
