Mick Pearce kindly sent me the info from Roud on Child materials from 1946 to 1986. As near as I can tell, there were only two additions to Child ballads discovered in America in oral tradition since Coffin/deV. Renwick in 1963. Both are Robin Hood ballads, #122 and #136:

ROBIN HOOD AND THE BUTCHER (Child 122 Roud 3980)
Book
Wolfe, Folk Songs of Middle Tennessee (1997) pp.23-24
Roake, Herbert
USA : Tennessee : Clarksville
1950 (19 Dec)
Boswell, George

MY NAME IS ALLAN-A-DALE (Child 138 Roud 3298) {"Robin Hood's Delight"}
Book
Roberts & Agey, In the Pine (1978) pp.65-66
Whitaker, Sam
USA : Tennessee : Oak Ridge
1957
Tompkins, Katherine

I have also taken a quick look at Richie's site and did not discover any additional information at this time. Richie, please correct me if I am wrong. I was getting pretty cross-eyed at that point!

I also looked at some Broadsides on the Bodleian site. I checked from 1750 to 1780. I found five copies of "Lord Thomas and Fair Elinor", one of "The Famous Flower of Serving Men", two of "The Hunting of the Cheviot" and one of "Queen Eleanor's Confession", and I think one of "The Jolly Beggar". The dates were uncertain on some of them. This was not a comprehensive survey of the 18th century Broadsides at the Bodleian. There may be a larger number prior to 1750. And I don't know about the end of the century.   There were not as many as I had expected in the middle.

I am beginning to get a very rough sense of the situation in England and Scotland in the 1700s, as Steve sums it up: " by the 18th century they were the province of antiquarians and those much further down the social scale. What survived by then was more the province of old nurses and the poorer tradespeople, those who couldn't afford to buy the current sheet music and go to the theatre." The question is whether the antiquarians were really tapping into the oral tradition. Until I can get some sense of the extent of the oral tradition in the 18th century, I don't think it will be possible to have any sense of what could have "come over here."