Hi,
SAP CUA cookbook recommends to use "Proposal" for parameter IDs. But it's still possible to use different option. Check note 395841. It describes PRGN_CUST parameter that allows you to control what happens in central system when PID is not defined there. So you can distribute PIDs from central system. You still need to figure out how to automatically propose values based on user's position. Have you checked if that new BADI is available in your central system?
Cheers