Shellfish Demi Glace? Is there such a thing?
In a word, no. That being because overcooking fish and shellfish will impart a bitter flavour and demi is generally cooked and reduced for many hours.
Again that being said the basic process of making a lobster or crab bisque would result in a rich seafood stock.
Crushing / pulverizing the shells makes all the difference.
Here is a great how to:
More Than Gourmet has a product called glace de fruit de mer, it is a combination of fish and shellfish that is quite good it has some added gelatin I think and natural gelatin from fish bones. It has a nice flavor and the aroma of a nice lobster stock. You can make a reduction of the lobster stock that Mila mentioned and use that in small quantities because it can be strong.