I finally succeeded at doing some subtle joints breaking with the Biped. There is an option to control joints separately in the animation rollout: it’s called Separate FK Tracks. You check “arms” then you get a true and simple FK system (you’re still bound to use quaternions though). It then went much better to animate this kind of movement.
Here’s the result!
http://www.jsnicaise.com/flv/ask/febuary/20100211subtleP.f4v http://www.jsnicaise.com/flv/ask/febuary/20100211subtle.f4vtime: around 1 hour