The Pace-egging song most likely evolved from the play itself. All it really is is a device to introduce the various actors. Although we don't have sufficient records to take specific mummers plays back beyond the advent of print something similar must have been performed in medieval times. The plays as we now know them were spread by little printed chapbooks published in their thousands in the north of England. Some of the better-known ones seem to have been simplifications of an earlier printed play 'The Seven Champions of Christendom' which of course includes St George.