I'm in Texas, and the sun is truly fierce in July and August.
Looks as though for ATmega168/328 maximum ratings
Operating Temperature.................................. -55 C to +125
According to the RFM69 datasheet, the absolute maximum temperature it can tolerate is 115C.
So, aside from the battery, what else might fail? Isn't it usually a capacitor? Are they all ceramic on the Moteino? I don't know enough to ID what they are made from or have a good guess as to what their upper temperature limit is for continuous use.
I picked 150F as a hopefully worst case, only because I've read that some asphalt roof shingles can get that hot under the sun, because that's the only top of mind reference that I have, imperfect though it is. Polished metal isn't very emissive though, so it might get just as hot or maybe even hotter, so it's just a ballpark figure.
It's frustrating to shop for consumer gear. For instance, I'm now realizing the picture I posted above probably doesn't even have PIR, even though it says it does. I say that because I found what looks like the same thing on Amazon, and it makes no mention of PIR, and there's no obvious visual indication either that it might be there.
I thought the metal would be preferable to plastic, despite the emissiveness problem, if only because I've seen plastic get yellower and yellower after hours of sun exposure accumulated on some expensive Optex PIR sensors I once had. So, I could address the emissiveness by painting it, but then that's more work for me. Or, I suppose I might just as well paint a plastic enclosure, and then it wouldn't yellow. However, in both cases, that's me doing the work and not chinamation, which is where it should be done. Furthermore, anything that isn't metal or painted will probably fog over under sun exposure, including the epoxy enclosing the solar cell itself, plastic lensing, etc., since it's not behind glass. Argh, so often it seems there's just almost never any consideration whatsoever given to product longevity...