You could also put a clamp-on housing stop a few inches up the downtube from the BB and run the cable through the stop and then through a short piece of cable housing that is long enough to run under the BB.

It will serve as a cable guide and keep the cable from rubbing the BB shell. I am running this sort of thing for both the RD cable and rear U-brake cable on my RockHopper and they work well.

Otto