Well, I just learned something, but what does “control” the IP mean? If they are only validating a single address via http then presumably you could just use an Amazon elastic IP as long as it resolves. I doubt that letsencrypt will support that but I would be interested to know. If they do then yeah, you could presumably set up the instance using the IP as the name, but I don’t know why you would want to. Apart from the fact that it would be hard to remember, could change at some point, screwing things up, it might work. I suggest OP do the necessary and report back accordingly.