Seiza is great for working on lengthening the spine and starting to use the pelvic crease to generate power...I think.....
Good stuff....Shirata Rinjiro apparently had some seated internal stuff...
Maybe Alan Beebe could comment....
Just as you say Jeremy, it all starts in seiza with the bottom sinking and the top rising and all the other postural adjustments of seiza. It is the zero before #1. That then moves to kiza, then to hanmi and finally forward movement. That's all tan doku dosa out of kiso kunren training.
Also there are the kokyu ho breath meditations as presented in "The Way of Harmony." (as an aside these three have been found to help people just starting with Buddhism to ease into zazen)
Angier sensei has a remarkably straight spine. He once attributed it to his exposure to one of the atomic tests for what it's worth. Rob might be interested to know he also compared notes with a noted Chinese stylist back in the 60s. See his article on AJ or EJMAS.