As far as I can tell Corsair doesn't have any advice on this topic. In this specific NZXT H500 case which way should I place the power supply? Follow the advice above and let the heat radiate up into my video card or turn it upside down and let it cook?
That's because it's not a problem.
You're worried over nothing.
This particular Corsair PSU is not made by Seasonic. You can mount it fan pointing down.
Ever since HardOCP reviewed the original RM and it failed in their incubator which has no airflow when the PSU fan is not operating, Corsair has made sure the thermistor that operates the fan is on the main PCB in an area that is the hottest when there is no airflow.
A lot of PSUs put the thermistor for the fan controller on the top of either the primary or secondary heatsink, which is closer to the fan than the main PCB. So while this is good at capturing the temperature of incoming air, it's not good at measuring the temperature of the air closest to the main PCB.