> It is very difficult for me to achieve that posture while sitting in a chair - the keyboard and mouse need to be in your lap, basically.

I found that I could manage that posture with a sufficiently adjustable chair and height-adjustable desk. I adjust the chair until it provides the correct height off the ground (feet touching, hips at the right angle), then adjust the desk and the chair arms until the chair arms go just over the edge of the desk, allowing arms to rest on them and use the keyboard naturally.

(Also, I use a USB ThinkPad keyboard, so my mouse is on home row, not off to the side.)