A little experiment I tried rigging some wings using spring constraints. Tunneling makes it hard to use but it's a start I think.
Playing with the thought of testing with cloth simulation using soft bodies attaching the vertices to a rigid body skeleton. Not sure though if Bullet could handle this. The cloth simulation in the demo just attaches one body to the end of a towel type object but this would be 3-digit fingers between which the soft body would be spanned. Would this be theoretically possible in Bullet?
EDIT: Expand the video comment for a full explanation of what I did and what is what.
EDIT: This video
contains another test run of the same. Look around 1:45.