Why not? Think about the miles per second that Lucas' mind must need to race to develop some of the ideas that he does. I'm sure the concept of Boba was developed well in advance. Besides, drafts of ESB would have needed to already be written by that point in 1978. He needed to have prinicipal photography done by early to mid 79 to be able to work in the SFX needed for the film. It was a much more time-heavy process back in 1978 especially when you're pioneering these special effects. It wouldn't surprise me if Fett was in his head pre-ANH but he just didn't really have a place for him in the finished product.