I'd add Masahiko Sato's early stuff -- Palladium, Deformation, Transformation, Samadhi -- to the mix. Also, Masahiko Togashi's records are generally a sure bet and very diverse. For being 'out there' they also are quite lyrical and melodic.
Mine is a good recommendation - have some excellent early sides. I'm not a huge fan of Sadao Watanabe; that said, the stuff he did with Charlie Mariano in particular is pretty heavy, and I have the feeling that he's made more records that I'd probably like that I have yet to hear.
Itaru Oki tends toward the freer realm but his amplified trumpet recordings (with small groups) are excellent.
Another pianist, Takashi Kako, was mostly active in Paris but made some nice dates with Claude Bernard, Mototeru Takagi, Kent Carter and others.
I think most of the stuff in my listening pile is pretty abstract/free/rackety but if any of that fits the bill I can drop some names. Homefromtheforest has the more straight ahead/modal Japanese stuff on lock, as well as the free stuff.