xp_fileexist Fails Even if Mapped to a Local Folder

Apparently the undocumented xp_fileexist procedure really doesn't like mapped drives. I knew about the headaches involved with using this proc to read network drives (permissions issues are usually the problem), but I expected it to return correct results for a locally mapped drive.

To see this, map your C:\Windows folder to, for example the W: drive, and then run this script:

-- Demonstration that xp_fileexist really, 
-- really doesn't like mapped drives - 
-- even those mapped to a local path. 
DECLARE @FileName VARCHAR(128)

-- Local disk folder succeeds: 
--    File_Exists = 1 
--    File_is_a_Directory = 0 
--    Parent_Directory_Exists = 1 
SET @FileName = 'C:\Windows\Notepad.exe'
EXEC xp_fileexist @FileName

-- Map a drive to  \\MYMACHINE\c$\Windows  as drive  W: 

-- Local mapped drive FAILS: 
--    File_Exists = 0 
--    File_is_a_Directory = 0 
--    Parent_Directory_Exists = 0 
SET @FileName = 'W:\Notepad.exe'
EXEC xp_fileexist @FileName

If there's a workaround for this, I'd be interested. (Yes, I could be the 1,000,000th person to write my own CLR to do this, but I'm hoping for a better way.)