I think if we're going to stick to canon and not ignore Peeta's leg, then Haymitch would feel obligated to volunteer for Peeta regardless of what Katniss had to say. Although I imagine that she still sees Peeta as an innocent, and would want to keep him safe. No matter how Haymitch ends up in the games, it will be bittersweet, but I think both he and Katniss will agree that Peeta will need to stay safe. I could see the two of them devising their own plan for the arena and leaving Prim and Gale out of it thinking they'll be protecting them too, but that's just an idea. For the sake of the story and the emotional impact it's going to have, let's let Haymitch volunteer. 

And that would be fantastic! :3