The indexing the original poster wanted has been done - for French songs. "Le Clé du Caveau" was a vast metrical index of French songs (from both folk and art genres) compiled by a large committee in the first decades of the 19th century - I think the definitive edition was in 1838; more than 3000 tunes. It's an amazing achievement and it's quite extraordinary that nobody thought to do the same for English or Scots.