I'd always assumed that that part of his technique came from the playing of John Martyn. John Martyn was a great innovator of guitar technique. There was his percussive style, his experimentation with tunings and later his use of electronics along with the acoustic guitar. The way Nic used tunings and percussion were different but back then everyone was experimenting. The guitarist who led the most was Davy Graham but for the combination of picking and percussion I'd go for John Martyn.