Well my initial thought was a PCIe card, but it could likely just as well be an SD or such if set up right. And yeah, you can get an approximation of it using _nix and clever mounts.
But i was thinking about having the initial OS sit on an actual ROM chip with a flash chip next to it on the same board/die.
What would be the negatives to this approach?
I have been doing this for many years and have still not found any.
Currently I prefer RO space embedded in the kernel for the "core" OS and mfs/tmpfs overlay for a RW, larger "additional" OS.
If RW portion crashes, the system falls back to the RO portion.