Probably not, just due to the fact that the map has to be carefully balanced, and a fully working AT-AT would seriously unbalance things. When we finally get around to introducing an AT-AT to a map the map itself will be designed with the AT-AT in mind so you can get the maximum gameplay use out of the walker, and its adversaries will have ample resources to fight it.
Having said that, watching an AT-AT trudge its way through a canyon would be pretty cool
. Not going to happen, not the least because of all the walkways and whatnot that'd get in its way, but it'd be cool nonetheless.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.