Not sure how NAFTA would work, but I venture that Directv wouldn't want to pay what CBC would want for national carriage. What would be good is if CBC did something like BBC America (but without the reality sh*t).
I'm not even sure if Directv could carry CBC as a local like the cable companies do in border areas. The rules for cable are different, same reason that where I live cable customers get affiliates from 3 cities, Lansing, Flint, and Detroit, - and a couple Grand Rapids too. Directv gives only one. The laws make them play by different rules.