[megameter]

Supine posture isn't really all that static if you're awake and on a surface at least as hard as tatami - you can shift around on the floor much more than in a chair, especially if the monitor can adjust a bit with you. Add a pillow anywhere and your entire force distribution changes. I would be a little more suspicious of the hammock idea, or soft beds. If you're sinking into it, it's obviously immobilizing.

Floor desking is my go-to for maximum variability though. It just needs "enough" monitor height/size that you don't hunch over to peer at the text. The rest happens naturally.

In both cases I use only a wireless keyboard/trackpad combo now. When I started out doing this I quickly realized it was the peripherals that were the problem.

[frereubu]

I see what you mean in terms of movement, but lying down is also not a natural position for a human body except for the duration of sleep. I mean, you might be able to avoid bed sores, but you're not using your core muscles in the way that the body is evolved for and your joints aren't being used in a way that will keep them in tune. It's a bit like astronauts losing a lot of general body tone and having to go through quite a bit of conditioning when they return to earth.