Okay.
First, I would not call it a national policy. I would call it a Canadian or a federal policy. And I would try to make sure not to place all the provinces on the same track or within the same norms, because we don't know for sure what will work best.
Yes, I would legislate, but I would focus first on areas of federal jurisdiction. There are plenty to play with.
Second, in terms of a role for the federal government, I would focus on measures that facilitate the exchange of information, the development of knowledge about the indicators of poverty, knowledge about policies, exchanges between provinces on best practices, and so on and so forth.