I use corny kegs as fermentors. Made my own air locks using a standard gas side quick disconnect and basic plumbing parts. I typically set them up like a blow off tube...just run a hose from the gas side to a small container (plastic bottle) containing a water/sanitizer or water/alcohol mix. This way the pressure relief vavle is still in tact...just in case.
Relying on the standard pressure relief valve as the primary means of venting will build up way too much pressure (about 125psi IIRC), unless you manually vent it...which would be very inconvenient during active ferm. I do sometimes leave kegs in secondary w out an air lock, but manually venting them every few days is all thats needed.
Just ordered a "spunding" valve, which is basically an adjustable pressure relief valve, for naturally carbonating in a keg. Traditionally, but with older tech obviously, this technique was used to carbonate lagers over a long period of time. I plan to use it for this purpose during lagering or secondary and continue to use airlock/blow off tube during primary. I dont think a spunding valve would be a good idea for primary because it is more likely to clog (its only designed for venting gas...not trub from really active ferm) and there is no reason to pressurize during primary.