I think I figured out how to handle the crankcase breather. I found a corrosion-resistant industrial breather.

The inside diameter is a perfect match for the outside diameter of the stock breather tube. I epoxied the breather to the tube and then tidied it up with some heat shrink.

I positioned it so that whatever happens to blow past the breather filter has a good chance of being sucked toward the air filter, which will be oiled anyway.