Riffs that make use of open strings are the only reason I can think of for a capo on a six string bass, but some people need that sort of thing.
As Jim Carr said, capos made for a 12 string or a classical guitar are wider than ordinary capos, measure your neck at the point you will be using the capo, and see what one of the big online stores have to offer.
I knew a guy who played a single course 8 string, and he used a capo on a couple of numbers that his band played in an open tuning. He used two Kyser capos, one on each side of the neck.
