Thanks this is a great point. It's not really for lubrication of the cylinder it's mainly to keep the bores from rusting during long storage periods. Seems like most of the suggestions here have been for greases which don't quite fit the bill.
The steel pneumatic cylinder will not rust if it is purged with dry nitrogen. It may be necessary to purge the cylinders , valves and plumbing, and cap off the supply and exhaust ports when the machine is put into long term storage.
This is a common method to protect machined or ground surfaces when grease cannot be used.
Your concern about grease ignition in a expansion cylinder is misplaced. In order for combustion to occur you need a ignition source, a vaporized fuel, and a oxidizer. All three components are missing.
Cylinder oil, for example, has a flash point of roughly 450 deg F. Your expansion cylinder is operating with gas at 1000 psi from a storage tank at ambient temperature. When the gas enters the cylinder through a fast acting valve it expands and drops in temperature. The oil film on the cylinder wall is, at worst, at room temperature. There is no vaporized fuel and there is no ignition source. Diesel engines require a fast compression cycle to generate the high temperatures required for ignition. They also require a fuel atomizing nozzle.
