As suggested in my earlier comment, in the absence of a Rotation input node.. (Why isn't there one? Maybe it's coming, unless there's some semantic hitch?)..
..You can instance a single-face proxy on one of the vertices of each of your instancers. That will be created with the instancer's rotation. Its face-index, when realized, should be in sync with the point-index (when the instancers are converted to points).
You can pick up the
Normal of the realized proxy, to get the Z-axis of the instancer, and then rotate the proxy by 90 around its local Y to have its
Normal point down the instancer's X, and extract that vector, too.
A 2-stage alignment can generate a Euler rotation which can be used as the 'Rotation' with which instances are created:
I've only included a 'Factor' input in the group for debugging and illustration. The group is probably better off without it.