Another feature of the playing of Nic Jones, John Martin, Bert Jansch etc. style is the RH falling on to the strings to damp them with a percussive noise; often followed by a syncopated or of-beat thumb stroke. Along with the frailing stroke, it seems to fit into a rhythmic groove, where something has to happen on a particular beat that doesn't seem to want a picked note. To me, it seems like a physical thing; once the rhythm gets going, it's almost instinctive, like a drummer deciding what to hit on which beat; so long as it's something. Like Howard Jones above, I don't see it as a banjo style, just something guitarists have developed along a parallel path. I hope that makes some sort of sense.
Richard