Ok, I won't raise the hand saying I'm dynamics guru,  and maybe my bad english makes me think what you are especting, johnpf, is to demostrate it's imposible, if that the case then ignore this post, again, it may be my bad understanding of english and if I'm wrong please forgive me, and this may help you or may help somebody else, this worked in most cases for me, and I'm shure it can be done way better and eficient.

Please note that I use a really bad dynamic dress for P5 Judy, I'm shure that with a better one with more polis it would have worked better.

Look how I set the scene in the first picture, the cube will be my chair

then move to frame 30 and set character pose the way I want in 2nd picture, and most importan I move and scale the cube to the right position and size

note in picture 3 how I use an small space between the char and the sit.

and the picture 4 is the result after 60 frames, with some more frames, it may have look better, the same adjusting the constrain group for the top area.

Hope somebody find it useful in same way. :-)