To my delight, I was directed by my hotel's concierge to Suzunami in the basement of Tokyo Midtown (Roppongi) when I asked for kasuzuke, black cod marinated in sake mash and grilled. Suzunami is a Kyoto-based shop that sells prepared kasuzuke for grilling at home, and their shop in Tokyo Midtown has a small restaurant in the back. It was exactly what kasuzuke should be: sweet, oily fish, boozy from the kasu, grilled to a nice char.
The menu consists of 5 set meals, each featuring kasuzuke and various sides, ranging from around Y1200-Y2000. Last order is 7:40 pm.