To evaluate an idea for a product it might help to look at the market success of similar products.
The
Truvelo NeoStead has similar features: 1) twice the normal capacity (2x6+1) of a normal pump-action and 2) the ability to choose from which mag tube rounds shall be fed.
Market success? Uh, I read an article about it a few years ago but haven't seen it since. Though that is probably more due to gun laws and import regulations.
Personally, if I were to buy a shotgun and the price were not much higher than that of a Benelli Nova, I'd buy that NeoStead.
If your device offers twice the capacity without making the shotgun overly front heavy and four feet long, it could have market potential.