In a way, it's not up to other countries to come and tell the Canadian government with whom they are going to negotiate. Currently on the international scene, other countries assume that it's the Canadian government that negotiates the treaties. In that sense, your bill is useful, but it's not so much as a result of the Constitution of Canada as it is the result of the customary practices of the international community. It might possibly be seen by the Canadian government as interference, if a government directly negotiated a free trade agreement with, for example, the province of Quebec.
On March 10th, 2011. See this statement in context.