Is the intent to let distros customize the appearance of DRM panic?
My point, ultimately, is that the kernel already shovels data into places the user can see. The features you're proposing exist, at least if I'm understanding what you're asking for. Panics will soon be shown to the user via DRM Panic. Userspace can already get early pre-initrd boot logs up to the kernel starting.
The one case that the kernel cannot cover is between ExitBootServices() and PID1