Sigh. While one is always free to define coordinate systems any way you want, it really complicates things to use coordinates that differ from 99% of the rest of the world! [A seminal paper in the eye-movement literature drove me crazy until I realized the Author was using a left-hand coordinate system, perfectly legal, of course, but confusing, as I kept getting the opposite sign from his results].
So I'll go back to my suggestion of doing as much as possible with Rotation Vectors to describe the orientation of the body with respect to a "base" orientation (which we can take as "standing upright, normal to Earth's Gravitational Field", the usual Z axis). If you now express change of body orientation as the derivative of the Rotation Vector, the problem reduces to having derivatives that depart "significantly" (to be determined by your study or modeling) from the Vertical Axis.
As I learned it, Yaw, Pitch, and Roll come from ship or airplane motions. There is always a question of whether the coordinate system is fixed on the Object or on the World, but I'm going to beg the question by assuming that the Object is more-or-less "upright" (the boat, for example, is in calm seas with its keel down). Yaw is rotation about the Vertical Axis (either Earth Vertical or Object "vertical" when in the upright position). Roll is rotation about the Object's "axis of motion", i.e. along the boat's long axis (as that is the direction it moves normally), while Pitch describes the movement of this "axis of motion" in the (Earth-) vertical plane. Note that the Roll, Pitch, and Yaw axes are always at right angles to each other. In the Usual Convention, Z is vertical, X is "axis of motion", and Y is at right angles to X and Z.
Bob Schor
