If you have a grinder, hold the cable close to the head with one hand, and lightly press it against the grinding wheel. Rotate slowly, and you'll reduce the OD of the barrel enough.

Most MAFAC brakes use a derailleur cable as a straddle cable. It routes through a "thimble", as show in item #59 in the picture below. They're typically just a hair big, my method, above, makes them fit easily. Regardless, it's a good idea to put a little grease around the barrel else they can seize up over time.



I wouldn't recommend drilling out the part when moding the cable is so easy to do.