Jon Hamm would have been good 6-7 years ago. A bit too old unless they're going for an end of career Batman.
Unless the producers are dead set on a "year one" Batman (and given this
, it sounds like they are proceeding with the idea that Batman has been operating for several years before the team up):
Hamm is only three years older than Bale and looks no older. He's approximately six years younger than Downey. If we can have a near-fiftysomething playing Tony Stark there is no reason Hamm can't play Batman.