Mounting a BB cable guide
 

I have an old centurion with no cable guide for the bottom bracket. I want to be able to install a Shimano RSX bottom pull dérailleur, but I can't figure out a good way to install a cable guide so I can run the shifter cable under the BB. I know I could drill and re-tap the BB, but I'd rather affix the plastic to the BB shell without permanently altering the frame. Any ideas?

Center column, mid page.
http://www.gaerlan.com/bikeparts/fra.../brazeons.html

Faced with a similar challenge on the Capo, i.e., installing a newer Campag. front derailleur without a cable housing stop, I simply continued to use my original downtube cable housing stop which was designed for traditional over-the-BB routing. I ran a piece of cable housing from the downtube stop under the BB and left the distal end open. It works like a champ.