It's not included because there's no good reason to switch from internal to external sync on the fly
What are you doing?
Evil things are going on you know
+1 I want to do this also. (for different reasons)
As a matter of fact as a general rule every button of a software should be mappable to access to the status of being a "good software"
I was surprised first time I saw some couldn"t be mapped > like the switch button between arrangement and session...
a save and undo mappable button could be cool also when you're standing up with instrument in your hand & that reaching for the keyboard of the computer seems suddenly a perilous move...