Yeah, the best ToDs could do is trigger the rule ONLY for the configured window, which goes down to a 30 minute resolution. We have the idea to create a "threshold of 1" type rule with a time over threshold that could be broad enough, but it's not really surfaced in the way the console works today.
While I could think of a good way to only fire a rule once in a set hour using some other events, I can't think of how you'd tie that to clearing the group.
I think you have us stumped. I'll ask around for other ideas.