A japanese planing beam is not knee high, nor do they sit on it. It
has a sawhorse-like brace on one end called a trestle (waist high) and
the other end is braced against the wall (knee high). It is used with a
pull motion plane, moving from the higher end to the lower end. The
worker is standing or leaning during the planing motion.

I think your idea of a knee high bench and planing while sitting is not
a good idea. I've seen pictures of Japanese man sitting on the floor
and planing but its with a very small hand plane doing some trim work.
He is using a pull style plane.

Bob