I don't really have a lot to offer in the way of route suggestions, but if you have questions about anything between, say, Marquette and Seney, feel free to PM me.
Are you limited to pavement? M-28 is probably your straightest shot across the UP. You could loop around on M-123 to see Tahquamenon Falls. That's a long loop though -- I think about 60 miles or so. Near Munising you could take a bit of diversion into the Pictured Rocks National Lakeshore. Going west from Munising you cuold take either M-94 or M-28. I'd go M-28. I see a fair number of loaded touring bikes going that route. You get some good views of Lake Superior. M-94 is all trees.
Sounds like an awesome trip. Have you begun already?