In creating a banjo for a lefty, I decided to tunnel the 5th string. I realize that this would prohibit the instrument from ever being set up right-handed, save carving a new neck. Anticipating that this banjo will be around long enough to potentially be in the hands of a righty, I decided to add dual 5th string tunnels.
The unused channel does not get in the way, negatively affect tone, etc; it only increases the instruments versatility. I also left the fingerboard a constant width to the nut in order to facilitate.
Here is a view of the brass tubes used for tunneling:
Here is a view of the brass tubing exiting the fingerboard just before the 5th fret: