EDIT: I think it need to further clarify what I'm trying to do:

I have a forearm rig setup in Maya using a wire deformer to control the twist. The wire deformer is on top of a skinCluster. The wrist joint has an extra angle attribute called "twist" which is linked to a parent joint that is at the same location as the wrist but maintains the orientation of the elbow joint. The twist attribute is also connected to the wire's dropoffLocatorTwist[1] attribute. With the dropoff at 100, the wire twists the forearm to infinity, and the joints still represent the "pose" of the wrist and hand. To compensate for over-rotations from the skinCluster, I added a duplicate wrist-hand hierarchy and connected the twist to its rotate x. Connecting the worldInverseMatrix of the duplicate hierarchy to the skinClusters bindPreMatrix attributes effectively negates any rotation of the wrist/hand joints around the axis of the elbow bone.

Now I want to push it further. To make it less of a hassle for animators, I want to remove the extra twist attribute and interpolate between all three euler rotate values of the joint itself to generate a twist value. Assuming the animator is setting values either directly in the channel box or simply relatively rotating the wrist, those values can easily go over 1000pi (in degrees). IF the poly mesh is dense enough, the wire deformer can easily perform a twist along a curve to that magnitude without any artifacts. The problem is how to interpolate the euler rotations to get a single angle value accurately representing both the pose and the twist. I've tried interpolating with the curves tangent and wrist matrix by multiplying each euler value by the dot product of the tangent to the euler rotations accompanying row vector in the matrix, but that doesn't work fully:

twist=rx\*(tangent dot row1 of matrix)+ry\*(tangent dot row2 of matrix)+rz\*(tangent dot row3 of matrix)

Particularly, it doesn't like rotations around the Y axis. Can anyone tell me why that is and how to properly decompose an euler rotation to a twist angle without constraining the twist to -pi <-> pi?

PREVIOUSLY ASKED:

I'm using Maya wire (curve) deformers and want to find a twist value along the tangent of the wire based off the euler rotation of a joint. Joints in Maya are really transform matrices, but are composed in a fashion that a user can input the maximum float value possible as a degree value for any euler rotation component. I want to take those euler values (x y z), and combine them in such a way that the result is a axis-angle rotation where the axis is the first row of the parent matrix (or the tangent to the wire), and the angle is an unbound twist value around that axis, allowing for unlimited flip-less twisting of a mesh. The problem is I can't find a way to interpolate the x, y, and z values so that the resulting angle only represents the twist and nothing else. To put it more simply, I want to find the 'x' rotation value for a euler rotation where the x-axis has been rotated and thus is now represented by 'y', 'z', or something in-between all three components. Is there any way to do this without losing the value of x to a lesser but equivalent value (ex 270 == 90, but we want 270)?