I've been waiting for someone to come along and say "I did this!"
Okay so I didn't do this, but I have seen several folks where I work create some stubbed AWS services to facilitate testing. I'll have to ask them if this turned out to be worth the (gargantuan) effort.
Edit: Looks like there are a couple[1] instances[2] where people have done this to some degree. Many of them are very old, though. The one built by a team where I work only implements SNS/SQS as well, so there's a pretty big gap for other interfaces. It seems like the recommended way to do this is to just use something like EasyMock or whatever and stub rather than have an actual API implemented for integration testing. I'm not sure I'm satisfied with that answer; on the other hand, it's substantially easy for a custom implementation to wildly differ from the actual AWS API behavior... Seems risky to base integration testing results on that?
[1] https://github.com/p4tin/goaws
[2] https://github.com/unbounce/yopa