I use msgbox2 extensively for decisions by the user. My current app has about 100 of them! So adding an invisible panel will be very tedious.
I've figured that tapping outside the box returns -3 (same as DialogResponse.CANCEL) and I only had to make a couple of modifications to the app to eliminate strange behavior!
As far as I know msgbox2 is modal, but tapping outside the box returns the same as tapping the "cancel" item. It doesn't make sense to me (my logic is that tapping outside the box should do nothing), but I guess it is what it is and we just have to sometimes live the the nuances of Android!