This is a great thread!
I love to use different techniques to get different sounds/feels out of the guitar. I use this same (back of the fingernails) strumming technique for certain kinds of music and it works really well, almost better (or more expressive) than a flat pick.
A few years ago I'd seen a close up of Paul McCartney playing "Blackbird" on an acoustic guitar, and I noticed his picking hand ... he was using this same technique and he only used his thumb and index finger. All these years I wondered how he got such an unusual picking groove in that song and it finally dawned on my that he was playing it much simpler than I thought. He also uses the same kind of approach on the song "Mother Nature's Son", and it creates a similar groove.
I saw Jeff Beck in concert a few years ago and I couldn't believe the variety of colors he would get out of a cranked-up Marshall and Strat, but then I noticed that he was playing with his fingers, this is where he gets so much of that expressive tone, by using his fingers to get just the right kind of attack out of those notes, muting, etc.
--Jim