Fantastic clarifications as well. I think that once this develops enough, we might actually have enough for a few pages in the official documentation!
That one I’m not 100% sure about, but what I do know is that on some of my testing machines that run Qubes OS, VMs have been able to read (but not write to) the bare metal BIOS.
Pure speculation, but maybe it depends on the hardware configuration?
I will play around with some of my HVMs to see if I can get TianoCore to read directly from the BIOS and get the OEM splash…
That’s the only way to fly 
Correct, and Qubes runs multiple instances of DRM and KMS.
That sounds a lot like what a stock Qubes OS dom0 already has in the majority of cases.
This would likely depend on hardware configuration, I would imagine.
Some GPUs (and other PCI devices, for that matter) register themselves as multiple PCI devices to the kernel, and they don’t particularly like to be separated and compartmentalised. AMD integrated GPUs are a good example of this, because they don’t like to be separated from the other devices that “cryptographically verify the integrity of the hardware in some way”, essentially causing a resume from S3 sleep to “lock up” the entire GPU 
But in any case, there’s only one way to find out. Please let me know if you need access to any testing machine (actual hardware, not virtualised) via KVM over IP. I’d love to help you guys out in any way I can.
So then, I’m guessing this would all be able to be taken care of by the dom0 initramfs, right?
It would be interesting to see what hardware the Xen hypervisor doesn’t actually give to dom0. Xen definitely does keep some hardware for itself, but whether that would impact whether plymouth can get read access to that buffer, well there’s only one way to find out 
On some of my Qubes OS machines (basically anything made by GPD, because they used tablet display panels that are hardwired in portrait orientation in their hardware), the framebuffer needs to rotated 90° at boot, and GRUB doesn’t seem to have any difficulties doing that.