I think that is the block drain just aft of the exhaust manny below #4 cylinder, but like you I couldn't get mine to budge at all and was wary about breaking it or stripping galled threads, so I just skipped it. Not sure it's even necessary for routine coolant draining, but even if so may not be much point to that if you'll do a coolant flush eventually anyway.

FWIW, I've found Free All (now stocked at O'Reilly's) to be a more effective penetrant than PB'laster for loosening corroded fastener hardware.