Not sure if this is what you are looking for, but the "Maps" application included in the $20 "January update" does have a nice, but limited offline routing. When you define a route, Maps downloads all of the route points as will as the zoomed-in map data immediately surrounding each map point. When you shut off WiFi, this data is retained for offline viewing. Functionally, the route is presented as an overview map zoomed out showing the start end end points. The UI has arrows that you use to step through the route, and as you do so, the screen zooms into each route point displaying the rout text and the zoomed-in map for the route point. And it is completely usable offline.
Presumably, you could map out a lengthy route that covers the area you will be traveling in. It could be tedious, but it might work.
Alternately, if you have the "January Update", the Mail application lets you view attachments, so conceivably, you could email to yourself downloaded map images, .PDF, or Word files and then view them using Mail's Multitouch zoom/pan viewer.
Both options are a bit of a kludge, but it might work.
Oh, and I have addressed this on other forums, and realistically, your best bet might be to just go to a good bookstore and buy one of those plastic-laminated fold-out detail maps of Tokyo. Not high-tech, but pocketable and reliable....
