PoPPo_G: "In an earlier post about Pre John Deacon Queen, John S. Stuart mentioned a recording by the Famous Jug Band which was produced by Roger Taylor..."
First: I think that should be Pre Roger Taylor Queen, not John Deacon.
Secondly: Essentially, what you have written is correct. The problem is that Roger's role is uncredited.
The reason for this is that he was involved with "Queen(ish)" material at the time, and viewed his involvement with a traditional folk/jug band as detrimental his musical C.V.
He appeared at the sessions - as you correctly suggest - as support for his (then) girlfriend Jill Johnson, but ended up on the tape as vocal support also. (Background/backing vocals). I believe that Roger added his "falsetto" high ranges.
His role as Producer/assistant producer is somewhat more muddied, so, whether he worked on the whole album, or a few isoltated tracks remains unclear.
This information was given to me first hand by two sources. One of which was Jill Johnson (who now lives in the USA), and the owner of the original reel.
I may point out that "The Famous Jug Band" recorded much of their material in a Truro church hall - before moving into the studio - and that some of these "rehearsal" tapes still exist.
Evidence of other uncredited work from Roger around this period included the "Dangerous" track by Eugene Wallace and his contribution to the "Larry Lurex and the Voles from Venus".
"Listen to them. Children of the night. What music they make."