Yes, physics is against us. Chemistry sucks, too. Do modern steel strings get rusty even before they wear out? Used to happen to the cheap affordable ones back in the day.

I don't think there's something special about resonator guitars. Nothing can last as long when exposed to elements as it would in perfect indoor conditions. It's just yet another risk you take every time you go to the pitch.

Typical tricks might work, too. Strings are surprisingly good at getting hot when direct sunlight hits them at the right angle. They lose their tuning, you retune them, and all these changes of tension are hurting your instrument. That's easy to solve, at least in part, by picking a spot where you turn away from the sun to face your audience. In this particular case, you also won't be annoying people by reflecting the light in their faces by that shiny aluminium plate.

Rain, on the other hand, is a real disaster. Seasoned wood covered with lacquer shouldn't react to a few drops too much but quick and uneven thermal contractions under heavier rains may cause problems, at least in the long run. I tried using thin plastic films as a temporary shelter but after a few days in my pocket they were all creases and holes and hard to unfold. If you find a plausible solution please let me know.