You might also want to check out Telehouse they have a lot of locations in Japan which they should be able to refer you to one of their clients that could do single U colo out there.
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You will find it hard to deal per U colocation if you contact telehouse coz basically they simply do not do per U and do not support you in English (yes, even telehouse Tokyo). Also, they don;t really want work with you if you are not registered as a company in Japan..etc
Best choice for you is to find other companies that have colo space at telehouse~
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