Of late, I spend a few minutes when I first sit down doing what you suggest, letting attention go to the area of my sacrum and its movement vis-a-vis the ilia. I also rock back and forward, sideways, and observe the rotation that naturally occurs. Then I think of the basic Alexander Technique instruction, “Let my neck be free.” etc. Once I’ve adjusted and settled in this way, I put my hands in the mudra, and start my zazen. I also use double mats below the cushion at home, or even bed pillows.
Since I started limbering up in this way, I can sit comfortably in half-lotus for two rounds of 40 minutes, with kinhin between. Not just bearably, actually in comfort. I didn’t even know that was a possibility. (1)
The part about “let my neck be free”, that’s interesting to me.
In my writings, I put forward Bartilink’s findings about “pressure in the fluid ball of the abdominal cavity” and support for the lower spine, including his observation that activity in the muscles of the pelvic floor and in the muscles of the abdomen and chest is resposible
for the “pressure in the fluid ball”. Sometimes I find that the activity in the transverse muscles carries up into the neck and head as well, so maybe there’s also support for flexion and extension in the neck when there’s “pressure in the fluid ball”.
Sometimes transverse muscles at the level of a particular vertebrae are a part of my breathing, and maybe I’m more aware of my neck and head when they are. I can’t really say that I’m familiar with Alexander Technique instructions (“let my neck be free”, etc.), but I do try to relax the thing that enters into where I am.