I think genetically engineering animals to be braindead is itself the unethical action. If we're willing to sacrifice efficiency we could emulate what animals go through in the wild. They live a life full of socializing and emotions and then slaughtered for meat at the end.

Is it ethical to genetically engineer animals (e.g. people) to be braindead?

I've known some (otherwise) vegetarians that ate bivalves, kinds of the same thing.

I'm not sure if this is a useful thought experiment.

Why not? Animal welfare is a primary reason why some people don't eat meat. Making them braindead should alleviate the concern for those people, but I have a feeling that the same people would be against it.