Is it possible to use another output for radio?
If yes, you can put a mute:<output> on the button to get out of the radio function like this: "MUTE;AUX||EXIT"
Can't test it, have no radio, yet.
EDIT: You will need a "RADIO||MUTE;AUX" to unmute again
Or am I completely wrong? I never used that function, but I remember having seen something like that somewhere. . .