> If the :stream syntax is not FS-specific

It is, I believe, as I alluded to in the comment.

> `\\?\GLOBALROOT\DosDevices\X\Y:Z\A:B` is harder to parse

As in, this is impossible to do statically in the general case - those names aren't guaranteed to look like that. See the note I had added about mount points. Remember C:\mnt can itself be the mount point of a volume instead of a drive letter. (Junctions present a similar problem, but at least for those, you can make an argument that they're intended to look like physical folders, and treat them similarly. With mount points, you might not have that intention - you might be just trying to go over 26 drive letters.)