By the way, Sn4ke_911, I am a member on GAF for almost 3 years and almost never posted here until today. But seeing your massive thread on MGS prompted me to get into the discussion.
I am among the minority who thinks Hayter is in it and have done some research on it as well. I came across several things which indicates his involvement but I will post just two things.
Check out this video:
www.youtube.com/embed/-Se5UHIjIfc
Now this is where it gets rather interesting. On July 30th, 2011 David Hayter answered a few fan questions.
Q. Are you working on any Metal Gear Projects?
A. I can't tell you if am working on any Metal Gear Projects.
Q. Are you disappointed that the Metal Gear Solid series is over?
A. Umm...Is it over?
Two points to be noted here. He does not want to talk about any future Metal Gear Solid game he might be working on which is kind of odd since he could have outright denied his involvement. Secondly he hints that the series is not over. Of course, he might be talking about Metal Gear Rising Revengance in his second answer or he must be aware that a new game starring Snake is being worked on.
and the second thing
https://twitter.com/DavidBHayter/status/283312270015082496
Now this is where things started getting rather obvious to me. When the teaser trailer for The Phantom Pain was revealed, the masked man in video sounded just like Kiefer Sutherland. A fan asked whether Hayter has met Kifer to which he tweeted that they met but their voices were too growly to understand each other. If you read between the lines, Hayter is teasing that Sutherland will be involved in the game after all sounding growly is a characteristic of Snake's voice. Again at this point nothing was ever confirmed by Konami or Kojima about the potential voice actors.
I still have a lot to share but will do later.