DanS, thanks for posting the photograph. I've found something similar at my local hardware store. I may try two or three of them on top of each other for a little more filtering effect.
There's a local business that manufactures stuff (sleeping bags, jackets, etc) from Gore-Tex and they were kind enough to give me a couple of small samples for free. I talked to someone in the warranty department to get it. Interesting stuff. If you put it up to your mouth it is hard to blow much air through it and of course water won't go through at all. I stretched the Gore-Tex sample over a cup of boiling water from the microwave oven. A room-temperature soup spoon held over the fabric readily condensed moisture. Wierd.
Because the Gore-Tex will clearly keep all of the dust/dirt out I've decided to try that experiment first. I suspect I won't like the results but am going to try it anyway. I've duct-taped over all the seams in the sensor case and taped a piece of the Gore-Tex over the sensor holes. I've mounted the sensor with the Gore-Tex facing INTO the airflow since I don't think dust will be a problem and it probably needs all the help it can get to circulate air/moisture inside the unit.
This same sensor was tracking well against another one in a second nearby FARS so I should be able to see what changes...
If I'm not happy with the Gore-Tex, then the brass mesh is next...