While I don't think they should have locked both DLC missions out entierly until after the endgame, I do agree that forcing you to take Hackett's transmission right after Horizon was very jarring. A better option would have been a simple email from him asking Shepard to get in contact ASAP. Then when you go to your quarters there'd be a prompt on your desk that you can activate to initiate the call. That way you could have chosen to talk to him right away, or put it off till last.
Actually, one of the things I kind of want included in ME3 is the ability to choose to call someone and have a conversation from the comm room. I mean, in ME2 wouldn't it have been nice to have been able to just walk into a room and speak to say, Anderson, Liara or Wrex without having to fly the damn ship halfway across the galaxy?