May I ask what’s the use-case here? Or, to put it in another way: Are you “only” asking to get a “read-only” notification about undo or actually gain the option to “interact” with undo?
If you’re asking about read-access (i.e., a simple notification without granting scenegraph editing rights), this should be covered by Support for history commands (as you then could just detect undo by comparing the history on update())…