The Watersons' site claims that Billings is the "author of this piece" which probably means "composer of the tune." This we knew. it doesn't say "author of the lyrics."
The Billings scholars on the dscussions@fasola mailing list say that the words of the "second verse" are not mentioned in "The Complete Works of William Billings" - and therefore almost certainly not by Billings.
I found the lyrics to "Rights of Conscience" - is this the right one?
"Millennial Praises Gospel Hymns" Published by the Early Shakers in 1813 Rights of Conscience Hymn #33