We use Canon DR-2580C scanners and had the same issue with greyscale as the only option thru the AC scanning interface. However, we used the CapturePerfect software that came with the scanner and set the default to be black and white thru the software. Then after we did this, when we went back and used the AC scanner interface, there still was not a "black and white" setting listed in the drop down box BUT if you hit the "default" button at the bottom of the scanner window, "black and white" would magically appear, so at least we found a work around. Not sure if the same will work for you.
