After getting sausage working, I decided to re-use the relevant code to add... sushi!
The cheese syndrome added some fun to an otherwise pointless element of the game. Why bother with pastures, livestock and buckets when all you get is +1 food? In that same vein, aquatic animals seem useless. They don't interact with your dwarves (though I added CRAZED to sharks and carp to try and change that) and, even if you go to all the trouble of draining the ocean/river to secure fresh fish corpses, aren't any better than yak meat. Hence: sushi.
Big fish have their own STANDARD_FISH_MATERIALS which is identical to the common vertebrate template, but with a cloned MUSCLE definition that can produce sushi via MATERIAL_REACTION_PRODUCT. Like sausage, it's an automatic reaction that should come as a pleasant surprise to the player, rather than a micromanaging chore.
I sat down with a nice seaside embark to test this all out. I built a filter trap lined with cages and glass terrariums, killed everything that moved until a Coelacanth spawned (which took a while) and after some unfortunate savescumming finally got it to flop into my drain trap before it could leave the map. While I had considered keeping it as a pet, it turns out that a bug causes aquatic creatures to air-drown in cages of any kind, while land creatures are immune to the effects of water...
The end result, however, was a meager pile of sushi and a heaping stack of fish organs, still no better than yaks. Perhaps it'd be worth it to reskin/rename some fish organ meats to "flank" or "fillet" to better give the illusion of the all-meat fish feast. Further tweaks will be tried.
EDIT: Single-count stacks of organs aren't worth renaming. Fish fat, however, is something of a contradiction - though tallow can already be cooked, trading fat for the "all-meat-fish" and effectively doubling the sushi count seems like a great compromise. It'll just be a matter of renaming a custom fish fat that gets processed to sushi...