What are the requirements for ventilation on a typical PoE switch? Obviously the switch has a fan, so it need some airflow, but will I be setting up for failure if I install a PoE switch in say, a fully enclosed j-box?
This will be largely dependent on the switch and it's own requirements. Your best bet would be to contact the manufacturer of the switch directly and get the details from them. There are actually quite a few PoE switches on the market now that do not have fans, this can change the cooling and ventilation requirements quite a bit.
It is also worth note that we do offer an 8-port PoE switch that comes in a wall-mount enclosure. It includes an outlet for the required power feed in a voltage barriered compartment, and full patch panel and all the required termination connectors. The model number is SNB-8.