Whoops,sorry, it's the Minato-ku Sports Center I was asking about. I played there last year and was curious whether it's open now. I believe the tables are on the 8th floor (18 or so tables). It seems like groups with pre-existing reservations can play there, but it's not available for open play...
I was there Monday morning and Wednesday from about 1 to 4pm. I did not see any Caucasians playing. I definitely would have stopped by to strike-up a conversation if I had. On Monday, I did see a "foreigner" dressed in an overcoat with some kind of big employee ID tag hanging from his neck...
Even if nobody from this forum is around at that time, you should be able to find somebody to play.
I'm visiting Japan now, and thanks to TWI's posts, I've stopped by, solo, and had some very enjoyable practices there.