More on the G2's Above 4G Decoding support. There's no BIOS setting for it, yet GPU-Z shows it as enabled. Many other Haswell systems are known to lack the option - and in those cases, GPU-Z agrees. It deduces the capability for Above 4G support, so there might be something to it. I started thinking, could the G2 BIOS keep it always enabled?
This led me to read more about PCIe memory-mapping. In short, PCIe devices have two-way traffic with the CPU and each other. MMIO controls all that by assigning memory space and addresses for all the devices. Legacy systems have a memory address range of 4Gb. With UEFI, devices can be assigned to memory above that (himem and config.sys, anyone?).
This thorough article studies Haswell's use of MMIO, and should thus apply to G1/G2.
So, on Haswell and newer, UEFI checks early if the MMIO is above or below 4Gb. Certain registers take hold only with MMIO >4Gb. With MMIO <4Gb, Above 4G Support is unused (yet it remains to be deduced by GPU-Z).
If Above 4G is always enabled, the next thing is ReBar. Again, the G2 BIOS has no option for it. Modding it is dicey, due to $SIG verification identifiers in two encrypted, non-UEFI data regions.
Out of the two, Above 4G seems crucial for Ampere support. This is because the card will always need proper MMIO. ReBar is a secondary function, which adds performance instead of compatibility.
This leads me to think the G2 (as well as G1?) could boot to W10/Linux with an Ampere (Ada) card. The Linux kernel would even allocate its own BAR for the GPU.