Is there now or will there ever be an option to choose whether or not a particular application has priority over an incoming phone call?
iOS always gives priority to the phone
No and doubtful.
Use the feedback links.
Doesn't Single Application Mode do that?
That's about the only way I suspect that a Developer created application is going to be able to take over a device enough for a phone to stop being a phone.
Perhaps you're asking: What is Apple iOS Kiosk Mode? - Amtel, Inc.