Since this subject is near and dear to my heart (I am always looking for a quieter way to generate our parked on-board air) I am following this thread (and it's twin on BNO) with some interest.
What about one of the Firestone "Riderite" DC air compressors? If you hook it up to an auxiliary tank it would supply just about everything you need and they are pretty quiet.
Dallas, this looks to be completely manually controlled. IOTW, no compressor control based on pressure -- it depends on the driver to operate the compressor when the gauge drops below the needed PSI. I doubt, by itself, that this would be a good solution for an air toilet. A pressure switch could be fitted, I suppose, to automate the operation.
Look at the silent aire site for airbrushing studio compressors real quiet
These all cut out between 40-55psi. Jack's toilet requires 60-65psi to operate.
Also, on both of the above suggestions, note that the toilet requires about two gallons of air at 60psi for each flush. So you need a fairly good sized tank, and these little low-volume compressors will run a long time bringing such a tank up to pressure.
Under normal circumstances, it's probably a long time between flushes, and this would not be much of a liability. However, I know from experience that there are, ahem, times when one needs to flush the unit several times in a row to clear the hopper. You may not want to wait, say, five minutes between flushes to build the required quantity of air.