This is good, thanks. But a much more interesting problem I haven't seen a good writeup for is how to interpolate smoothly between quaternions at different times. Quaternion slerp has jerks (C_0 but not C_1 or C_2) at the keyframes.
The abstract mentions it: “This paper gives one answer by presenting a new kind of spline curve, created on a sphere, suitable for smoothly in-hetweening (i.e. interpolating) sequences of arbitrary rotations.” And the final punch line is section 4.3, then you can work through the details in the earlier sections.
Basically it extends Hermite splines to Quaternion splines using the Lie group operations.