Has anybody used one of these "SureFire Mossberg 500 / 590 Shotgun Picatinny Rail System M70" Its a quadrail foregrip, not the surefire with the built in light. I've never seen one and can't find any reviews or write ups about the product. It's a little pricey but I don't mind that if it works well and is a little different.
http://www.surefire.com/M70-Mossberg-500-590-Picatinny-Forend
http://www.surefire.com/M70-Mossberg-500-590-Picatinny-Forend