Low power strategy for dealing with spontainious bit flips durring sleep on AVR 8-bit?
Can you use two pins for the interrupt? That way you could have two PCMSK bits set. You can also set up both pin change interrupts, which covers PCIEn. That leaves only GIE. I don't think there's anything you can do about that.

My (admittedly vague) understanding is that cosmic ray-induced bit flips are more of a problem for SRAM and DRAM, which have weaker feedback in their data storage. I haven't heard as much concern about registers. Personally, I'd be more worried about your stack and global variables since you don't have ECC RAM.

Regardless, to get a true high-reliability system, you need an MCU that's designed for it. There are products available with redundant hardware and ECC memory. Unfortunately, they are neither cheap nor low-power. These sorts of MCUs are normally used for dangerous applications like airbags and medical devices.

My suggestion is to program as defensively as you can, but accept that you can't eliminate every risk unless you're willing to pay for it. The odds of your GIE bit randomly flipping are very small. Unless a failure is going to hurt or kill someone, that level of risk is not worth worrying about.