Uhm.. another idea (final idea I hope ).

I will put a checkbox button in the control panel as the ones on your screenshot without making other complicated key shortcut combination and let the users decide to enable/disable it whenever they want.
Now the last thing to manage is what to launch when you open the shutter, having to start an application is a little bit heavy and not so "quick".
I will look if I can do something better than a simple dialog, that, by now, seems to be the only possible (and maybe best) solution.