Wouldn't a better idea for this be that the USA annexes Japan before the Meiji restoration? One of the biggest basis for Japanese identity would be Emperor and the survival of the Japanese Imperial Family. Assuming, say, the Emperor and his family perished before Perry's arrival, that would leave a chaotic enough situation for US fleets to land, try and restore order, and eventually annex Japan.
However, since this is the post-1900 forum, I'm going to have to accept the limitation of annexing post-war Japan. The biggest fulcrum here is the destruction of the Japanese Imperial Family. Or leastways, a surrender scenario where the Japanese are too broken to even request the Imperial Family stay untouched.