take a look at the first video in the tutorials section… I found it very impressive how the bones system works… Maybe Synfig could have something similar? You just have to “draw” the bones, and attach the vertexes, and voilá, just move the joints : )
well, Genete is or was working on a bone system, maybe you already know it Bones GUI discussion
I already revised this software and while impressive on it’s performance, it’s intended more to work with sensors for real-time performing, unfortunately, it does not have any use for regular home-made animation.