You could peek at the widget indicated in your screen shot, at least when that MDI (?) frame is showing.
The window title at kLayoutPresentationBoss, IID_IDOCUMENTPRESENTATION also reflects the value.
Probably best would be the kLayoutWidgetBoss, with its IID_IPANORAMA, GetXScaleFactor() - and GetYScaleFactor()!!!
I just try to imagine how a document would look if these differ from each other ;-) and, even better, how to achieve that from the UI.
Btw, one notification is on kDocWorkspaceBoss, protocol IID_IWINDOW, command kUpdateDocumentUIStateCmdBoss.
Another one: there is a kDocWindowTitleModifyService ...
Edit: of course I should again mention IID_IPANORAMA, observable from kLayoutWidgetBoss subject.