Hi,
for all I know it's not allowed to call a popup or a message via BAdI "ME_PROCESS_PO_CUST" directly. Have a look at the documentation. I think there are some special macros to output a message but no solution for a business case as you described.
Further a popup would need input from user. That could be a problem in background processing if someone is using a function like "BAPI_PO_CREATE1".
Kind regards
Michael