Is it possible to perform cosine interpolation between two transformation matrices?
It make sense for the translation part, but how about the rotational part?
Yes, you can.
same as for the translations, you extract the 3 angles form the matrix, you have the starting angles and the target angles. Interpolate as you wish (sync-async, cos-spline-linear) and for each timestamp you can re-create the matrix (knowing the 3 angels and the 3 translations).
Advantage for cos-interpolation would be the continuity of higher order derivates (same as in the case of the interpolation for the linear axes)