I mute the 4th and thirds stings to get a percussive bass and play it sowewhat close to the bridge with the fingerpicks on. For an exercise try playing the thumb alternating between the 4th and third stings on the 1 and 3 beats. In this style the back and forth action with the thumb on the low strings delivers a constant, propulsive bass rhythm. Th banjo doesn't have the lowest bass range but I still dig the sound of it.
As always write me if you have any questions.
