How to lock PO document and change it from outside of SAP?

Requirement: After user save the PO. The PO will be locked from sap users. PO can be only updated by BAPI_PO_RELEASE/BAPI_PO_CHANGE outside of the SAP.

This is what we do for now.

We save the PO info in a add-on table to keep locking records (also publish data to a outter system). Then we activate user exist MM06E005 to check the add-on table and keep normal users out doing any updates. The updates can only be done by a outter system using JavaConnector(calling BAPI_PO_RELEASE).

Now here's my problem. BAPI_PO_RELEASE seem to be bumped into the user exist and locked like every other sap user as well. Is there any better way to do this other what I do now? Please advise...

Thanks biggy.