What type of sake should I use in the marinade?

You could also use sherry or Chinese rice wine in this, if you don't have sake on hand. ;o)

I have his cookbook and he's not specific in that either. So sounds like any sake you like since any flavor nuances will be covered by the miso. He does say if you don't like the taste of alcohol, he recommends Nikiri Zake. And his favorite sake is Hokusetsu, which is served in Michelin starred restaurants so probably insanely expensive.