The biggest fear compressor manufacturers have about Nitrox is a bad mixing stick running a slug of O2 through the cylinders. Next concern, not as big a worry but important all the same, is the faster degradation of oil and rubber seals. By the way ... Brownies is just a reconfigured Bauer compressor.
That's why Coltri says that their machines are only Nitrox compatible using a membrane system ... impossible to make a slug of O2. Plus, oil is now 100% synthetic from day one and is recommended to be changed more often.
That said, you should be able to use your compressor by shifting over to synthetic oil and installing a membrane system. A single stick can handle up to 10 CFM and make up to 40%. You will either have to use standard air reduced to 100-150 psi to supply the membrane (it takes about two tanks of air to make one tank of Nitrox) or have a low pressure compressor on board. Of course the LP compressor is easier but a good one will run you a few bucks and take up some room.
As for Nuvair, they are the world's largest distributor of Coltri compressors and they build all the Coltri Nitrox machines (mine too). There is just no one better for your custom membrane installation.