I know this works with the WinNT provider logged in with a local account but wkstn joined to target domain ... not sure about workgroup.

code:
$root = GetObject("WinNT:")
$domain = $root.OpenDSObject("WinNT://@LDOMAIN", "Administrator", "password" , 0)
Not sure if you can just replace WinNT with LDAP and have it work, worth a try ...

-Shawn