Hi Kevin,
As you said, you can Enqueue the table in ME_PROCESS_PO_CUST and Dequeue in ME_PURCHDOC_POSTED after insert here.
Still if you want to show some error(insert failed) here, u cant show it in ME_PURCHDOC_POSTED.
(SAP doesn't allow this bcoz it can lead to data inconsistency).
Even I am not sure of any better solutions other than these...Kindly share if you find any.
Br,
Maju