The main question is who holds the know how on how to enable/disable write protection and shadowing of the e and f segment.
Well this is chipset specific (memory controller).
And: Whether we need to care at all. Can't we just leave that space writable and rely on the OS not trying to overwrite the machine's BIOS? I would be surprised if any OS really tried that.
This is what I did exactly with the K8T890/VT8237. The chipset has the shadow ram prot bits even if the memory controller is in K8. But hey wrong setup of this caused DMA from HDD to fail if the physical memory target for DMA was in "shadow" memory range.
To sum it up. Coreboot should handle this. ADLO/LegacyBIOS whatever should not touch this. We can unprotect the shadows regions in coreboot. All we need is just sane memory map.
Rudolf