So this new rule IS in effect this year. I'm not sure if the Bombers have needed to take advantage of it.
From the article: "CFL teams have to start seven Canadian players at the beginning of each game, declaring a number on both offence and defence that cannot be reduced over the course of the contest. Under the current CBA, a veteran American player with three years of service on his team or four in the league may replace an injure Canadian starter - a rule that Hamilton has been accused of exploiting this season - but it appears their fancy substituting caught up with them in the regular season finale."