I agree with your suggestion. It is a problem. I frequently use a category called "admin" in a situation like that but it would be nice to have something mods could use..
Welllll, replying to a deleted post is not the reason for removal. If the message can stand on it's own and doesn't violate the rules, then it should remain.
For example: "I agree" makes no sense on it's own and therefore should be removed for "off topic".
However, "The company's phone number is 888-888-8888." stands on it's own and should remain.