Thanks for the responses, everyone. As all my award flights (NW or otherwise) originate from NRT, I've never had the opportunity to take an intra-Asian flight connecting through NRT on NW. From your comments, it looks to be pretty difficult. Still have no problems with booking award flights from here. In fact, I'm on my way to GUM on CO at the end of March.

And SPN Lifer, about that UA - NH code-share GUM-NRT you're interested in trying out, I love NH (except for their mileage program), but NH's GUM-NRT flight is not one I would want to take. Lv GUM 0525, Arr NRT 0800; Lv NRT 2200, Arr GUM 0225. Not my kind of schedule. On the bright side, in my searches over these past few years I've noticed that the fares are usually cheaper than the other flights offered (NW, CO, JAL), and there are always more open seats.