To be honest, I doubt that drainage will help: if there is a connection, it should be draining quite low from Pant Mawr; otherwise it'll come out of the bottom of Moelwyn itself.
The vein at Moelwyn seems to be quite vertical and there may be an issue with the connection (if there is one) being on the wrong side of a deep hole!
I'm tempted to suggest that the more interesting route may be into the top of Pant Mawr and then down to one of the lower adits - I doubt there's a connection to Fron Boeth, but there have to be some way into the upper levels of Fron Boeth which can be seen from the lower level.
Incidentally, I don't suppose anyone has a survey for Pant Mawr / Fron Boeth lying around? :angel:
Hello again darkness, my old friend...