Haven't been aware of this thread till now. I note that Dominic Behan's alternative verse has been overlooked:

He knew the route the Bags would take along up Watling Hill,
His heart it pounded like a flute he told it to be still,
He lay in wait by James's Gate and when poor old Bags came up
With his long knife he took the life of the poor oul gatherem up.

In my opinion, the idea of twang being a kind of sweetmeat, and a Twangman a dealer therein, is just prurient wishful thinking. It's surely a sexual term.