I play fingerstyle guitar, but I don't sound anything like Nic. Having said that, I don't think that Nic's style has anything to do with clawhammer banjo. He used open C tuning a lot, and a percussive technique which involved flicking the strings with the back of his nails somewhere near the guitar's bridge. This interview on Bright Young Folk should help a lot. Plus, Nic is a huge Bob Marley fan - probably because of Bob's unconventional time signatures.