"The objects must be indexed so that each object has a unique id in its patcher, and that id is to be reused by only a new instance created by the undo queue."
Which is code that should be added to PD retrofitted to the application
externals changes the number of connections which may or may not cause issues.
My point is that it is a non-trivial thing to add to an application not that it is impossible.
If you feel that you could add it quickly and easily then perhaps you should - I cannot deny it would be an excellent thing to have.
mark