Hi Maarten,
I think you can try the below enhancement -
CVDS0001 - DOCUMENT_STATUS01 => If the status for DMS header is "locked" status, then do not allow adding/deleting assignments records form the control tables like DRAD.
For more details, please check the SCN WiKI link -
Object Links in Purchasing - ERP SCM - SCN Wiki
UserExit list: Userexit for CV01N/CV02N | SCN
You can put a thread too in the right forum - https://scn.sap.com/community/plm/document-management
Thanks,
Arijit