What about using CotnrolTemplate for changing the layout of MessageBox, that should work?
And another question, I just found, that If I show two dialogs at the same time and click a button on it, then this click is propagated to both dialogs at the same time and both are closed simultaneously. This needs to be fixed as well. Ideal solution should be something like the system dialog, when two alarms are raised at the same time - I have to close the first and the second one... or at least I'd like to have something like this:
behavior, so I can await for closing the first one before showing the other one.