Other than more high-tech, the Audi V12 is none of those. I've also got to believe that it is considerably more expensive than the Duramax, which cuts considerably into your marketability arguments -- if a marine Duramax really isn't marketable, then a marine Audi V12 is probably less so.I would want something that is smaller, lighter, more high-tech, higher reving with less effort than a Duramaxx.
While it hasn't been as high-profile an effort as the racing Audi V12 TDI (which is quite a bit different from the consumer V12), Banks has been endurance racing (or at least preparing to do so) with the Duramax. They had transmission problems before they switched over to Pat's 6-speed, so maybe he can tell us more about how that effort is going.
In all honesty, it sounds like your reasoning mostly amounts to wanting to do something different, but cost/benefit-"bang for the buck"-"return on effort" arguments are currently hazy at best, and may fall on the side of the Duramax. If nothing else, I'd want to have something much more concrete than just your suppositions as to why marine Duramaxes are not on the market in quantity before deciding that such engines are not a viable option.
I really don't understand your comment about marketing the same base engine under different names. Yes, Mercury builds BBCs, as does Crusader, and many custom marine engine builders. Do you really think customers can't tell the difference between one of JCPERF's engines and a 496 Mercruiser? Or are you worried that you (we? who or what kind of company is supposedly building and selling this engine?) couldn't build a Duramax-based marine engine that is better than anyone else's?