You can take exactly that approach, thanks to the linkd.exe tool in the Microsoft Windows 2000 Resource Kit. Think of Linkd as a simplified Mountvol. If GUIDs are your style, you can use them with Linkd (although I can't think why you'd want to):
linkd C:\extradrive\\?\Volume{47c8cd01-1a1f-11d5-
aa7f-806d6172696f}\
What Mountvol can do that Linkd can't is list GUIDs—if you execute the Mountvol command without parameters, you get a listing of current GUIDs and the drive letters that map to them.
Linkd can also report what, if anything, is attached to a given directory. So, the command
linkd C:\extradrive
returns one of two responses, depending on how you created the mapping:
e
or
\\?\Volume{47c8cd01-1a1f-11d5-aa7f-806d6172696f}\
To disconnect the reparse point, type
linkd C:\extradrive /d
Finally, some words of caution. First, junction points give backup programs fits unless the backup programs are specifically Win2K-aware. For example, you can use NT 4.0's NTBackup program to back up a Win2K system, but NTBackup either won't back up or won't completely back up files or directories that exploit new-to-Win2K NTFS features, such as junction points or sparse files. Second, the free-space reports get a bit weird. If you run the Dir command on the C drive, you'll see C's free space. But running Dir on C:\extradrive shows E's free space.