I am in the same boat. A previous discussion mentioned appending it to the "closed" state to do so, but that was 5+ yrs old, and would not be a good solution for reporting or forgetful Agents.
So I would second a Request for Change for this feature. I'm not for sure the technical bits of how filters works yet (I'm new to it as well) but I could not find a good matching condition, if customer post then match, and if current status does/does not equal status-x (we have an "unassigned" status we made for brand new tickets, which we would not want to change to "Awaiting our response" when they reply to a brand new tix), then match.
Since the state changes from closed to open status, upon customer response, the back end components must be there already. I'll do some digging to try and figure out a workaround until they do implement such a feature.