The word "clawhammer" is not used correctly here. Clawhammer is a method of playing the 5-string banjo which cannot be duplicated, and still be clawhammer, on an instrument that doesn't have a high tuned string below the other strings. To play clawhammer, you use the back of your fingernail to play the down-beats and the thumb, a hammer, a pull, or a slide to play the off-beats. There are people who have retuned guitars, sometimes by putting an HO guage railroad spike at the fifth fret and replcing a fifth string with .010 string and slipping it under the spike. This makes it possible to play clawhammer guitar.
   I think what Martin is refering to is what is often called Travis Picking, Cotton Picking or just plain Finger Picking. This is an alternating bass on the Down-beats and the fingers any place they're needed.