I have read in some books and online that some parts of the country are closed to tourists. There is some info here on the official Myanmar tourism website, which lists “townships” that are permitted or not-permitted, but it’s not clear what this means for specific roads.
So I’ve posted some messages on some Lonely Planet forums about the viability of the route, and got some good answers. The most amusing one being “If you see someone waving cheerfully from the back seat of a limousine as you puff bravely along the potholes- it will probably be me :-)”
The route does seem ok, and there are people who have ridden it before. A few stories of police making people take the bus on specific sections. The route is pretty flat. Even though Mandalay is ~600 miles upstream of Yangon it still only 64m above see level. So there won’t be many hills.