Lighter, it is true that this aspect of song dispersion has not been investigated to any extent. It is an idea of mine that may be shot down by more study.
There are a few broadside collections in North America, but their contents are known only to a few, as the lists have not been published as far as I know, and certainly the lyrics have not been published. American Memory is one exeption.
Many broadsides have a tune specified, but this does not always help either.