Most of these solutions seem to be for guitar fingerpicking - UP-picking against the nail. What are people using for clawhammer or frailing banjo style picking, which is DOWN against the back of the nail?

My own, interim, solution to this problem is to trim down and file down a plastic fingerpick and put it on the opposite side of the finger from which it normally goes. Getting used to it is going to take a while. And, NO, this is not the same as using an Alaskan pik, which goes underneath the nail.

Some people use a metal pick similarly, and some of the old Minstrel books have patterns for the "stroke" style of picking. Both plastic and metal picks for down picking give a significantly different sound than the bare nail, which most people are used to for frailing.