Two years ago we too made dessert sushi, and luckily we remembered to take some pictures.
Thanks to Megan for reminding us about the project!

We built the sushi on a base of simple white cake, sliced to look like little cakes of rice. For wasabi, we used hard icing with a little bit of green food color added. We tried a number of different things for the seaweed (nori) wrapper. Among others, we tried several brands of fruit-roll-up kinds of things. The most successful of these was "Fruit by the foot," which exhibited remarkable stability at room temperature and atmospheric humidity, in marked contrast to some other varieties. Besides that, the striated green color was just perfect for the sushi. The toppings seen here include thin sliced twizzlers, miniature jawbreakers (as roe), and Swedish Fish. The neat rectangular cuts of "fish" are actually gummy bandages, detailed side down, that we picked up on a whim. These turned out to be some of the best looking pieces that we used, and were actually some of the best tasting as well.

On the lower right you can see our version of Futo-Maki, the big sushi roll. While it's possible to roll a cake up, we decided to start with round cake. To do this, we put some of the cake batter in a muffin tin, and edited the resulting cylinders to the correct shape. Color-selected Jujubes provide a good substitute for salmon eggs, and itty-bitty round sprinkles subsitute well for flying fish eggs. In the upper right, dried apricots do a pretty good fish imitation as well.

The little round roll sushi worked quite well with both the tobiko and the larger white (jawbreaker) roe. In the upper and lower right are two things that did not work very well: Using colored gum paste as nori, and using a gummi-crab as a topping by itself. While both of these seemed like they should work, the color and flavor were bad and worse. Overall, the sushi was a big hit, and we look forward to an excuse to make it again.