We have had many discussions about this:
The PSV is designed for out flow
The PSV is robust by design and routinely inspected
The PSV has a steam tattletale if the bellows has been blown
In 33 years, we have never had gasses from the flare over come the steam purge and the integrity of the valve.
No history of a PSV failure in reverse flow
We have downstream block valves they are car sealed open and not used for LOTO unless the PSV is removed for maintenance.
Our operators all have H2S montiors on while in the unit.