You could set up device-specific iMessage accounts and mark each as the Caller ID sender for its particular device. This is a little absurd, but it would work for enabling you to receive an incoming message on all devices, but have subsequent replies directed only to a particular device. For example, Glenn could set up firstname.lastname@example.org on each of his devices, so iMessages to that address would show up everywhere. But, if the Caller ID address for his Mac was email@example.com and his iPhone was firstname.lastname@example.org, responses to his iPhone-generated replies would come back solely to the iPhone. Similarly, when he initiates a conversation from a given device, his recipient doesn’t need to know that’s where it comes from; they just reply.