Usually chat moderation relies on having participants in the room flag things if they go wrong, so there wouldn't be a need to make a room a gallery room just because there's no room owner at the moment.
I think before you go considering measures as drastic as that, there's also the option of adding room owners in different timezones and adding redundancy (so that there's multiple room owners from multiple timezones in case one has to work or have a meeting) if the room really needs that much babysitting.