Basically I want to use the WPF Binding framework to "observe" a property in the data context, and when that value changes to call an event handler. I do not actually want to bind it to any target.
Sounds simple but from what I can see Binding is too coupled (to the visual tree and various other bits) to be able to use it flexibly.
Any thoughts?
You are correct that bindings are associated with the visual tree: they're about hooking UI elements up to data elements. So if you wanted to use a binding for this, you would indeed have to set it on a dummy framework element.
However, if WPF can observe the property then you can too. WPF is just using the data context object's INotifyPropertyChanged interface. So rather than setting up a binding, you can just cast the object you want to observe to INotifyPropertyChanged, and subscribe to its PropertyChanged event. Internally, that's all WPF is doing anyway.
(If you're concerned about lifecycle issues, WPF provides the PropertyChangedEventManager which uses weak references. Call PropertyChangedEventManager(dataObject, listenerObject, "WhateverPropertyYouWant") where listenerObject is the object you want to receive the change notifications.)