Hi Thomas,
I am very interested in seeing MakeHuman succeed, so I'll chip in my thoughts when I can. Unfortunately I don't use MakeHuman or Blender, and between work and family I don't even get a lot of personal time on the computer. So I'll have to limit my contribution to forum replies for the moment.
I tried to dig up my old rig that had shoulder deformation rigged up (in Maya), but i couldn't find the file. It could have also been something I did for an employer a while back, it which case I wouldn't have had the file on my computer to begin with.
Anyways, for the trapezius bone I was thinking of something that originates in the middle of the neck, and then points & scales its tip to the top of the shoulder where the clavicle and scapula meet. This will help keep the area between the deltoid and the neck from collapsing when the arm rotates upwards.
I snagged an image from the interwebs and did a quick paint-over to show what I'm talking about:
The green lines are bones and the blue lines are muscles/stretchy-bones. Shoulder placement should be a little higher than you currently have it, since the shoulder rotates around the center of the ball-and-socket joint where the humerus and the scapula meet. This is a little higher than center of the meat of the arm. The scapula attaches to the end of the clavicle and rotates from just a little off from the centerline of the body (but higher than your current L_shoulder bone). There is a little bit of give where the scapula and clavicle meet, but not a ton.
Usually it is good to have some additional bones along the upper and lower arms to help spread any twisting along the length. I usually do this with three bones. In the case of the upper arm I would have the top bone point down the length to the elbow but not twist at all, the second point down the arm and twist half way, and the third point down the arm and twist all the way so that it is in the same space as the elbow. This kind of setup allows you restrict twisting of the mesh to mostly below the deltoid and thus keep your shoulder cleaner.
For the pectoralis major you will want to use several bones, since the muscle originates along a long edge that traces the sternum and down across the rib cage, and then inserts into a small area near the top of the humerus. If you are just using straight stretchy bones, you may want the origin to be further towards the middle of the muscle (perhaps in a line that traces through the nipple) so that you don't pull the mesh through the rib cage when the arm reaches backwards.
Your lats and shoulder fan look like they would work pretty well; you'd just want to tweak them based on how the mesh weighting goes.
There is no simple approach to mesh weighting... you just get in there and do it. I tend to not paint weights since it isn't very accurate when you have a lot of bones. I like to assign weights directly to points based on which joints I want to affect them, and then smooth the results on some of the verts until i get something that works for me.
I'll see if I can dig up some older samples from my hard drive in the next couple of weeks. Before a couple of nights ago, I hadn't actually launched a 3D app at home since March! I used to rig a bit at previous employers, but now I'm a Lighting TD so I don't play around with character rigs all that much any more.
Cheers,
Michael
