Re Canadian route, Canalligators, thanks for the heads-up. Unless there is a Canadian equivalent of Adventure Cycling Assoc, it may be difficult to find out too much route information. I could reconnect with the NT, but I'm still going to look for options that head directly toward Montreal, if possible. Do you know of any good sources of info for possible Canadian routes?
Unfortunately, there is no Canadian nation-wide touring association. My general rule on Ontario is to avoid the busy roads. They seem to be quite allergic to paved shoulders (don't get me started on the Ministry of Cars, er, Transportation).
From Mackinac Island, I'd probably head south down the east side of the Lower Peninsula and pick up the AC Great Lakes connector across Ontario. Then use the Hamilton-Brantford Rail Trail to Burlington, and the Waterfront Trail across the north shore of Lake Ontario. In Quebec, consult the Route Verte website, the province has a wonderful network of long distance cycle touring routes. Otherwise, if you want to brew your own route, check out the MapWorks series, they have detailed maps of various regions, for much of Canada.
If Mackinac isn't a hard destination, you can go from the Twin Cities across Wisconsin on the Lakes Connector, take the SS Badger to Michigan (my favorite), then across Southern Ontario.