Now I shall tell all!
Methodism was founded by Saint Methodius!
The saxophone was invented by Saxo Grammaticus!
Seriously: this thread is putting out shoots in a number of interesting directions. Whatever may be the case for this particular song, the history of religion, festivals, and merry-making in the British Isles certainly correlates to trends in other areas of culture, such as music. A thread on this topic might be interesting, and well-received.
And there have been, in the English-speaking world, examples of songs used as signals or codes. Certain slave-songs come to mind. I can't think of any examples of a complex, intricate secret mnemonic such as is being suggested for 12DC, though. But a thread on mnemonic songs and verses (e.g. "thirty days hath September...") might be interesting.