Sorry, I don't know that detail. Microsoft does provide less capabilities in their OS when using V4 drivers. So it may be due to a MS limititation in the API they provide to driver developers.
I am not aware of any way to force mono via the driver. You can set a default, but the end user can override it. Two possible options:
1. Use the Xerox XML Driver Configuration tool to create a configuration file used by Xerox drivers, which has an enforced mono value. (You will need to manage the deploymen of the file to end user systems).
2. Use XWS to set a rule which enforces mono. So even if the user submits as color, this would get overriden at release time.