Ardodd Pwnc- literally ‘reciting/relating topic’ was the forerunner of the Whitsun Walks and Anniversary Services which were huge events in Blaenau Gwent and other Valleys areas right up until the 1960’s. Ardodd pwnc consisted of members from chapels in neighbouring communities e.g Tredegar and Rhymney, meeting at a halfway point to sing and perform verse etc. Does this tradition still continue anywhere in Wales, in whatever form.? It was very significant in the development of Nye Bevan’s character and beliefs, to name but one.
I wonder if this is the root of the “preaching services” that chapels sometimes have in the valleys. It’s an odd name for a service (after all, what chapel service doesn;t include preaching?), and it seems to have the particulars that you note - a number of local chapels come together to sing and hear readings on a specific subject.
In Llandysul, (and Cwm Gwaun, I believe) we have a special service on the “Hen Galan” - the old new year, being the 13th (12th? I can never remember) of January, the new year before all this new fangled Julian Calendar nonsense came into being. Apparently, it used to be a football match between Llandysul and Llanwenog (some 12 ailes away) - first to kick the ball into the opposing church was the winner (no other rules) - but after some deaths in a number of matches, the local vicar decided that a bible evening would be more appropriate, and so it was.
I like that it was only after “some deaths” (and not just the first death…)…you wouldn’t want to change things until you were really sure about a thing like that! 
Diolch Iestyn, I shall hopefully catch up with you in Llanelli at the Eisteddfod by which time some more on this culturally important tradition may have come to light.