I'm going to assume we're talking about the 47 Ronin here, yeah?
Hey, he tried Shakespeare a few times, why not try bein Japanese? After all, if it was good enough for John Wayne....
Oh, as an aside, most know about Keanu's role in "Much Ado About Nothing", where his non-existant "English" accent featured in most reviews, but my favourite story of him in Shakespearean acting featured him in the title role of Hamlet, I think in a Canadian production. First night on stage, he walked out... and promptly forgot his lines. Being Keanu, he reportedly said "Well, I forgot my lines... but hang on, cause it's going to be a blast!".
The teenage girls in the front of the audience adored him.