John, here is a link to over 1000 songs, Public Domain Music (http://web-helper.net/PDMusic/). It has an index to the songs in two books and then a scan of the sheet music. I have the second book, but not the first one.
(1)Master Pieces of Piano Music
(2)The Book of a Thousand Songs
Back to your original idea...it seems just too difficult...much like creating another DT! But I like the idea of links to libraries or on-line sources that have various collections. I think there are already several listed in the Mudcat links. Perhaps we could add to those, or reorganize them, or index them, or do something along that line.