Can the pyrophoric tendencies of a pyrophoric substance be suppressed in a pressurized environment?
Depending on the pyrophoric substance, there can be differeing effects from changing air pressure. Strange as it may seem, sometimes combustion is slowed at very high pressure. For example, ethane, which reacts explosively with oxygen at STP, oxidizes slowly in a series of steps at ~100 atmospheres (~107 Pascals).

That said, it depends on the substances involved, pressure and presence of catalysts or other substances. Aluminum in a ball mill produces pyrophoric dust, but adding graphite during milling suppresses ignition.