The My Documents directory location is stored in the registry at HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders in a key called personal.

The following script is what I use to remap this to u:/documents:

;; create some dirs:
u:
mkdir documents
mkdir outlook
 
;; Move My Documents folder:
;; delete current setting
reg.exe delete "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v Personal /f
;; create new setting
reg.exe Add  "HKCU\Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders" /v Personal /t REG_SZ /d u:/documents