While hinoki is impermeable when cut parallel to the wood grain, transversal sections are 6~8 times more vulnerable to water infiltration.
The traditional joint allows for full coverage of the plank head. Also, plugs and SUS screws can be inserted deeply in the joining plank, making the tub extremely solid and durable.

No, we will not make 45° mitered cuts. It is a weaker joint and is not a proper detail for a hinoki tub. I wish that tubs with mitered corners were not misleadingly called "Japanese tub"...

UPDATE 2008/06/20
We developed a model of tub which retain the constructive traditional details while providing an clean "minimalistic look". This was achieved with thicker wall planks. See also the examples below:
http://www.bartokdesign.com/japan/7-custom_ofuro/hinoki_tub_for_surrey_2.php

http://www.bartokdesign.com/japan/7-custom_ofuro/hinoki_tub_for_manhattan.php