One pack goes on at 400AGL, it's included in the performance criteria, including 3-eng climb. Nobody wears oxygen masks at BOG [8360], unless perhaps you're a heavy smoker and have difficulty breathing.
Pax 74s had been operating into BOG [AV- Avianca] way back in the 70s; bleeds are not kept off during the inital climb. It's an elementary certification criteria; if the airplane couldn't climb on 3-eng with pressurization capability, then it wouldn't be certified to operate at this high altitude airport.
By the way, 74s also operate into UIO - Quito, where the elevation is 9226 feet.
