It's unlikely they would release the Pre Order Exclusives as DLC, this would just be a swift kick to the genetals of those who went out of their way to preorder from a specific place so they could get that specific bonus.
Out of curiousity, when these Pre Order bonuses were released to everyone for other games in the past, how angry did those that did preorder get?
I had an exclusive multiplayer character in gears that was pretty cool. Used to get messages about it. Yeah, it was pretty lame seeing everyone running around with the same character once the dlc was out but it really wasn't a big deal.