I have never heard Bert or Martin use a clawhammer style on guitar. Clawhammer is a banjo style that usually uses the back of the nail on the beat and either the thumb, a hammer, pull or slide on the off beat. Much of the sound is dependant on the re-entrant tuning of the 5 string banjo. For this reason, it has recently become popular as a ukulele technique, since the uke also uses re-entrant tuning.