A very light application of bee's wax (sold as a lubricant for wood screws) on the cable portion that runs in the bb tracks works well. Once a month I also clean the tracks with a tooth brush.
I agree the exposed design is a poor design. The problem of ignoring the cleaning is that the performance of the cable is gradually diminished, and you don't notice it. I learned this the hard way, when I broke a shifter lever because I needed to apply more force. Clearly, the shifter should not have broken - it wasn't that much force- but I don't think the added force helped.
After fixing the shifter, cleaning the tracks and lubing with a small amount of bees wax the shifting is soooo much better.
Installing a piece of housing is not a good solution because I see the housing filling up with dirt. But I've never tried it, so I can't say for certain.
In any case, the bottom bracket tracks should be on everyone's preventative maintenance routine.