Maybe you can use additional in opening_hours >> Mo 07:30-15:30 || “24/7 emergency service”.
There is also a service_times key in wiki. Not invented for this but maybe usable?
Or even use a separate key as these are effectively separate services, e.g., emergency_opening_hours or emergency:opening_hours. However I suspect the service_hours key may be just as appropriate for the 24/7 service.