Hi astralclouds,
There should be no impossible things, but this is one close to impossible.
APM terminal at Yokohama is tucked well inside the industrial area; otherwise it would be possible to have a glimpse from the harbor tour boat.
Thinking outside the box, the way to lay a proper ambush for a vessel coming to Yokohama would be to wait for it at the entrance of Tokyo bay, in Uraga channel from Yokosuka side.
Also, knowing the exact time of arrival (or a very precise AIS data) of your vessel, and being very lucky with the ferry schedule, it would be possible to observe the particular ship entering Tokyo Bay from Kurihama-Kanaya ferry. Also, this would be the only possibility to use the phone camera, as all coastal ship spotting positions require 200 mm lens.
I hope I am wrong, and I'm waiting for more experienced Japanese spotters' input.
Cheers
Vlad