You all know the Windows Phone backstack right. If you go through some apps, tap the Home key after starting each app. Now tap and hold the Backkey to see the Backstack. Y
Following my comment, I've just tried this:
From there, I think we can safely conclude that the task switching and the thumbnail are handled entirely by native code. Therefore, there's nothing you can do.
I recently wrote a blogpost in which I discuss the ways I tried to hide data from the application snapshot. (You can read it here: http://corstianboerman.com/trying-to-hide-crucial-data-from-an-application-snapshot/)
The outcome: You just can't hide it.