Zinc plated fasteners are an option, they are fairly corrosion resistant and have the correct strength, the zinc coating is not as thick as galvanized and doesn't flake off. I'm not sure what grade to use on frame members you would have to do some research. Grade 5 is pretty strong and can take a little more flexing than grade 8 but I don't know which would be best for that application, plus they seem to paint well...
