Tailpipe fires are not hazardous as long as you have airflow through the engine, at least have the starter motor on. If that is not possible (PAX milling about the inlet after self evacuating) then cutting the fuel as quickly as possible will reduce the damage to the aircraft.
I don't recall an incident where a tailpipe fire alone, burned through an aircraft firewall although there have been several where the aft wing was severely damaged while on the ground.