Depending on your alarm system is there no way to be able to passcode that? I.e I have the Ring alarm system so I can arm and arm through alexa and use that to trigger other actions but to disarm you still have to provide a verbal passcode to alexa.
But to your original point, Echo speaks has a SetWakeWord exposed command, so theoretically maybe you could set it so that if your GPS location is not detected at home then a custom action is run on all devices to set the wake word to "computer" but then once your GPS returns it does the same to change it back to "Alexa"?
Not sure if this would work in real world scenario though?