There's no need to lube the outside of the housing because other than you sliding it through, it doesn't move in the frame. In fact, many owners of frames with this configuration find the housing to walk changing the loops at either end in ways they don't like. I have a friend who ended up putting a dab of hot melt glue at one end to stop the walking.

OTOH- if you ride in wet weather, the housing ports are entry points for water. This makes a weep hole in the BB mure critical than otherwise. Besides the weep hole, I'd consider a bead of silicone caulk around the top tube entry point as a weather seal.