All I have seen out of Microsoft only covers CPUID levels 0x40000000
as an vendor identification leaf and 0x40000001 as a "hypervisor
identification leaf", but you might have access to other information.
No, it says "Leaf 0x40000001 as hypervisor vendor-neutral interface identification, which determines the semantics of leaves from 0x40000002 through 0x400000FF." The Leaf 0x40000000 returns vendor identifier signature (i.e. hypervisor identification) and the hypervisor CPUID leaf range, as in the proposal.
This further underscores my belief that using 0x400000xx for anything
"standards-based" at all is utterly futile, and that this space should
be treated as vendor identification and the rest as vendor-specific.
Any hope of creating a standard that's actually usable needs to be
outside this space, e.g. in the 0x40SSSSxx space I proposed earlier.
Actually I'm not sure I'm following your logic. Are you saying using that 0x400000xx for anything "standards-based" is utterly futile because Microsoft said "the range is hypervisor vendor-neutral"? Or you were not sure what they meant there. If we are not clear, we can ask them.