'Source singer' is a construct of the British folk music community. A lot of the literature simply uses the term 'folk singer', but we've been around the block many times with that term, and it's entirely obvious why some felt the need to find an alternative. Personally I'm quite happy with 'traditional singer' but, when that too became ambiguous owing to the usage as 'someone who sings a traditional song', then yet another term was coined. 'Source singer' keeps the distinction between 'tradition' and 'revival', but some people dislike it because it reduces the Walter Pardons of this world to reservoirs of material for folkies to use. I'm not particularly attached to it, but I can see why it's there. If we're to reject it we're still going to need something.

Regarding singing style as a basis for selection, I know of no examples. Collectors recorded Walter Pardon's repertoire, music hall and all, because he had a large number of songs learned from his family, not because of the way he sang. Sharp certainly made comments on singers' styles, but this was not a gateway. There are recordings available of Country-inflected Traveller singers, which argues against that style having been filtered out. I've also heard field recordings of women singing through the natural'break'. Jim can no doubt tell us more.