> drive letters are essentially just a convention borne out of the conversion of a Win32 path into a NT path

CMD also has the concept of a current drive, and of a per-drive current directory. (While “X:\” references the root directory of drive X, “X:” references whatever the current directory of drive X is. And the current directory, i.e. “.”, is the current directory of the current drive.) I wonder how those mesh with non-standard drive letters.

They work just fine, as the drive-specific CWD is stored in the environment as a normally-hidden =<drive-letter>: environment variable which has all the same WTF-16 and case-insensitive properties as drive letters:

    C:\> cd /D λ:\

    λ:\> cd bar

    λ:\bar> cd /D C:\

    C:\> echo %=Λ:%
    λ:\bar

    C:\> cd /D Λ:

    λ:\bar>