...evidence that the appearance on a broadside... means that Fred Bloggs, professional broadside writer, wrote it more or less around the same date it was printed.

It's always going to be very difficult to provide a smoking gun for a lot of these songs, since most broadsides were anonymous, and in any case there's always the possibility that another version existed before the oldest known copy.

However, in the specific case of the 'Wild Rover' I was talking about earlier, we have a known composer (Thomas Lanfiere) of the 17th century 'Good-fellow's Resolution' broadside - which is very clearly a 'Wild Rover' precursor - and we know that this author specialised in writing moralistic 'Bad Husband' ballads of this type. That suggests that his is the original copy. It is exactly the kind of turgid doggerel that people have been talking about above, but following on from Lanfiere's original over the next two centuries you can trace a number of edits, in which bits of his ballad have been cherry-picked, rearranged and eventually assembled into something resembling a folk song.

That's one well-documented example, and I daresay it won't be possible to do that for all the 90%.