Earlier quoted context omitted.
Do you know of any off-the-shelf that are qualified as reliable at the temperatures of cryogenic fluids? (F9 LOX is believed to be at roughly -200 Centigrade, or around 70 Kelvin.)
I can't think of the manufacturer of the top of my head, but there are definitely off-the-shelf options. They are used on the LN2 tanks and dewars you see at a lot of electronics (and I'm sure other) manufacturers, and I'm pretty sure are standard kit for any cryogenic vessel. I think the bigger issue, as one commentor alluded to, is that you might not want to vent whatever is in the rocket straight to atmosphere.
Basically once the cryogenic fuel starts to warm up you have to launch or to empty the tanks, the tanks are pumped out into reservoir tanks which are usually below the launch pad and fueling or emptying the rocket is in general the most dangerous part of the launch/abort sequence. Temperature and pressure variance, vibrations, leaks, and a lot of moving parts if anything goes wrong it can result in a pretty big explosion.
Overall they do have ways to empty the tanks, but this is a controlled pump out of the fuel at pressures and rates that would be as safe as one can get when dealing with supercooled liquid oxygen and kerosene, uncontrolled venting is a big no no.
You can similarly see this on larger tanks of both inert and reactive gases, a small propane tank could have a PRV because a flame out is likely to be less dangerous than an explosion in a BBQ setting, a tanker won't have a PRV because if it vents several tons of propane the result would be as bad or worse than a potential or an actual explosion. LN2 tanks also have PRV's as long as they are small enough to be safely vented but as soon as you go into a large enough volume where venting it is no longer safe whatever fail safe you implement has to be controllable.