Jumping ahead a few versions, mostly because I did V19 — 21 in relatively quick succession and I forgot to make notes, so I don't quite recall all of the tiny tweaks I made. Sorry...
One thing I noticed about the previous versions was that I was keyframing too much, as odd as that may sound. I was keyframing the forward, backward and side-to-side tilting of the torso and hips on every key pose, which was getting me needlessly confused as my values weren't exactly planned and I'd often end up rotating it too much, or in slightly the wrong direction. As a result, the torso bent and twisted all over the place in some very peculiar manners and looked totally unnatural. It was only really noticeable from the front but if it looks bad from one angle then something's obviously not working and it's important to fix it, regardless of whether or not it will actually be seen from that perspective.
What I decided to do instead is simply set two keyframes for the rotation — one on each contact pose, with the torso twisted to the left and right accordingly. I then focused only on keyframing the Y rotation for the torso — getting him to lean forward and back as appropriate. Softimage then handled the interpolation for the sideways rotations automatically, calculating a fairly smooth movement for the twisting of his upper body.
I'm much happier with how it looks now. Once I started allowing Softimage to take care of the side-to-side rotation of the hips and chest it started looking much, much better and the torso has stopped wiggling around so much.
There's something really not right with it though. I'm looking at it and something just looks really iffy; much less 'sneaky' than, say, version 17. I compared the two side by side and I think that it's actually the leg - or, rather, the hip.
On the left is frame 19 - the 'up' position - from V17 and on the right is the same from V22. The leg on V22 is a bit straighter due to the reversed rotation of the hips, stretching the leg upwards. It's only a minor difference but it seems to be creating a bit of a 'pop' - he rises up too far and too quickly to be much of a sneak. I did try to rotate the hip downwards in favour of the leg in order to slacken it a bit but it doesn't seem to have helped much.
I think I'll try using the position of the hips from V17.