Undo Redo management in AIR

I've been looking at the undoManager and not understanding some basic
principles, obviously.

Situation, I have an AIR desktop app and want to include Undo/Redo in the
menus. When I do that, it overrides the typically expected behavior of
control-z and control-y, so I assume that I have to manage that myself now.
I've been able to handle cut/paste overrides by capturing the menu event and
using NativeApplication.nativeApplication.copy() paste() etc, but there's
nothing comparable for undo/redo. So that took me on a quest looking at the
UndoManager, which appears to be my ticket for managing Spark TextInput text
operations. However, if I trap the menu event for undo and look at the
undoManager.canUndo() after deleting text, it's always false.

I'm missing a step somewhere. Can someone point me to an example or a better
explanation of how to manage undo/redo in an AIR application, if you want to
override the built-in menu items?

Thanks.