Applying settings from a GPO to a mac
If you had an OS X server you could use MCX settings in OpenDirectory to achieve results similar to GPOs. As it stands, the best solution I could come up with is use of a commercial Mac-GPO solution like this or this.
That said, if you're open to non-Group Policy methods for managing this, you could change the location of user home directories, or even try mounting the directory (I assume it's on a network share) and then using a symbolic link to point Documents there.