Damnarty,
Was there any leakage before you carried out the oil change?
From your posts it seems that the oil only flows when the engine is running. This suggests that your hole connects with one of the oil drain ways in the cylinder head? (From your photos it is not completely clear as to where the hole is.)
Does the hole have a smooth bore, or is it threaded?
If the latter, it is possible that, as you suggest, a threaded plug may have been lost, but I cannot understand why the hole should be there.
As an aid to understanding, I will attempt to attach a copy of the Iveco Daily 2.8JTD lubrication system. In particular take note of the oil drain ways from the camshaft area to the sump.