so the plot thickens...
is it possible that the national anthem is sung AFTER some games, like championship cup games?
i did have the feeling that the final death aria in many operas might have something to do with it, but the fat lady in that case sings all through the opera, not just at the end. that doesn't seem to match the feeling of the expression which seems to connote that the fat lady sings only at the end.
the other thing i was wondering is whether there was some music hall connection. unfortunately i know nothing about music hall shows. is it possible the phrase was borrowed for baseball from a source like this?