One of the ILM guys in the old docs mentioned the script merely said something vague like "and the band began to play", and little else in the way of a description.

The song wasn't a last minute thing though. In Classic Creatures and Making of a Saga you see Sy Snootles being built at the same time the others were made. Lucas apparently came up with idea of her singing when he saw the her in the ILM creature shop. The song had to be finished enough to have playback on the set. (The English version anyway.)

I think Sy went off to powder her nose in the other scenes. Didn't she show up on the sail barge later?