It looks like the '::tk::mac::Quit' callback does not exist in the Apple-supplied Tk 8.4.7 in OS X 10.5 and 10.4, although it does work with the Apple-supplied Tk 8.4.19(?) and 8.5 in 10.6 and with a current ActiveState 8.4.19 on 10.4 through 10.6.  However, instead of that, it looks like overriding the 'exit' callback will work fine on all of them.  In a simple test, I'm able to intercept the Tk app quitting, post a yes/no message, and then terminate the Tk app by destroying the root.  Next I'll try integrating into IDLE.