Kendall, I can think of lots of singers who can carry an evening of unaccompanied singing. I would agree with you about Roy Harris (Burl on Mudcat) who I haven't seen for some years now, but I should see him this Saturday at the "Remembering Fred" event.

Although some of these do/did use accompaniments on the odd song, they are all capable of working without instruments. Of course some of these are now sadly deceased or no longer performing.

The Watersons
Johnny Collins
Graeme Knights
Dave Webber and Annie Fentiman
Artisan (Superb harmony singing)
Tom and Barbara Brown
Ian Woods
John Foreman
Mervyn Vincent
Charlie Yarwood (esp with Dave & Annie and his ex-wife as "Beggar's Velvet")
The Young Tradition
English Tapestry
The Dead Sea Surfers
The Songwainers

This is only a very small selection, but perhaps our UK folk audiences are more used to the concept than US ones.

I will be running part of the singaround at "Remembering Fred" at CSH this Saturday, and I'm willing to bet that the majority of singers will be unaccompanied.