Leave the PO Release Workflow as BLANK.
Use help of Security team, to restrict access to PO release TCODES (ME28 and ME29N) for all users Plant-Wise. The PO will go for release as per strategy but ONLY users authorized for a Plant will be able to see their PO and release it. They will not see POs waiting for release if they do not belong in their authorized plant.
Authorization should be maintained on basis of all PO parameters:
Release Group + Release Strategy + Plant
Check with security team.