I know on most big jets there are hydraulic accumulators that do have a nitrogen precharge of around 1000 psi. This accumulator system is usually only on the brakes. The problem is, once you pump the breaks a couple times, the precharge is gone, so you need hydraulic pumps (either electric or mechanical off the gearbox) to recharge the system.

There are period of very high demand for hydraulic pressure on an airliner, which are when the gear is retracted right after takeoff and at flap extension/gear door retraction after extension on configuring for landing. The 767 and 777, in addition to electric and mechanical hydraulic pumps, have an air driven demand pump, which use bleed air off the engines and only kicks on during periods of high demand, or if one/both of the electric/mechanical pumps fail.