Hi Sylvie,
Right now our permissions are department/cost center based and not plan based. Thus, once a plan is open anyone with a department or cost center permission can see it. The only thing I can think about to work around this is to keep the plan in the new state or archive it and make the one BPO you want to see it an admin so they can view it. Things to be aware if you do this though is that this user can now change ANYTHING as they would in turn be an admin.
I would also add this as an idea in the ideas page as I am sure other customers have this ask as well.
Let me know if you need clarification,
Lauren