Coming in late (as usual), picking up on a few random points:
Another way of hardening the fingernails is to eat lots of Jello and/or take gelatin capsules.
At a Mariposa Festival one year I saw Kyle Creed frail using a finger-pick of his own invention. I've never seen anyone else use a pick.
To muddy the waters a little, there's a guitar finger-picking style called "clawhammer". My former brother-in-law, from England, played what he called the "Regency clawhammer", which he learned while working in Belgium in the '60s. I couldn't describe the style from memory, and he hasn't been my brother-in-law for a lot of years now, so I can't check with him.
I'm also under the impression that some people use "clawhammer" the way others use "melodic clawhammer", i.e., lots of melody notes, less rhythmic, and use "frailing" for the more "brushy", rhythmic style, instead of using "clawhammer" interchangeably with "frailing".
Lastly, does anyone else out there frail with an anchor, i.e., keeping the little finger on the head? By the time that I found out it was impossible (Pete Seeger's book, I think) it was too late to quit. You gain a lot in precision but lose in attack, volume and traditional sound. It's good to have a louder instrument to play this style. (Mine was made by Jake Neufeld and has an old-fashioned bell-brass tone ring.) It works well for the melodic clawhammer style.
The only other person I've seen frailing with an anchor was a bluegrasser trying to demonstrate frailing and, I think, remembering it imperfectly. Am I all alone out here?