Hi...
i think that through OKEQ and OKP1 controlling area is the permitted level.
To get the locking in terms of company code and plant you might think of designing some validations (GGB0) where bukrs and werks fields are available.
You might check on discussion
OKP1 - period lock on company code level
where they describe a note (that should be payed as a separate SAP service)
1697047 - CO period lock at company code level
Hope it helps..
Cheers
Panagiotis