How can I have arbitrary code executed in a User Control when a custom dependency property external to it has changed?

I'm not sure if I understood what you want to achieve, but if the problem is in getting one more callback/event to track changes on dependency property(or attached property) then this.

If you're using MVVM, keep in mind that your application should be your ViewModels, not your Views.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.