Read the Noteworthy Shipments.
Basically, if you're taking the mick, no it's not covered. And sending a human inside it has been done before (as has drugs, bombs and just about everything else), but is still taking the mick.
Thus still liable to seizure, arrest, etc.
"The packages constituting the diplomatic bag must bear visible external marks of their character
and may contain only diplomatic documents or articles intended for official use"
Given that the purpose of the bag is to move "The official correspondence of the mission", unless they'd tattooed their visa lists on him, I don't think you could class him as correspondence, hence you wouldn't be able to get away with just stuffing him in the bag.