Shibuya has a sushi bar and I think there is a good chance a solo diner can get a seat weeknights and perhaps early or late on a weekend, if you are lucky. Of course, if there are a lot of conventions going on, I cannot speak to that.

Everytime we have gone, be it weeknight or weekend, for a reservation between 6pm and 7pm, all tables were booked. I did not ask about the sushi bar