Putting a fixed AR stock on a shotgun tends to eliminate most of the advantages you were looking for when you decided to do an AR stock conversion in the first place.
The issue we as a California manufacturer had is we would have been unable to develop an AR style stock for a semi-auto if there was ever a chance that we might accidentally attach a collapsible stock to our prototype conversions. The moment we did that we would be breaking the law.
But that information on the web site is old. We are now in a position where we can get permission from the DoJ to produce "assault weapons" for sale to military and police. Indeed, we are now working on collapsible designs for the Benelli M4 (though not AR-oriented designs, since they are impossible on most semi-autos). Come the day we want to make prototypes, we'll use references from law enforcement and military customers to get the DoJ's permission to proceed.
However, even outside California, there are Federal regulations against combining pistol grips and collapsible stocks on imported semi-auto shotguns like the Benelli. If we made such a stock, our customers would all be military or law enforcement.