In a small saucepan, heat the lemon zest and lime zest with the sake over medium-high heat until the sake is evaporated. Set aside to cool.
In a separate saucepan, mix water, sugar, yuzu juice, lemon juice, lime juice, and corn syrup and bring to a boil over medium-high heat. Reduce heat to low and simmer without stirring until all the sugar has dissolved, about 3 to 5 minutes. Remove pot from heat and let syrup cool completely. Add the lemon and lime zest, then pour into a shallow baking dish or plastic container and place in the freezer. (The 6 cup capacity shallow plastic containers with a lid that measure 6 inches by 10 inches by 2 inches deep, and which you can get in grocery stores would be ideal for the purpose.)
Check after about 2 hours and stir the mixture. After about 4 hours, the mixture will have begun to set up, and you can score the surface by turning a spoon backwards and using the tip to scrape the frozen syrup into "ice curls." Repeat this scraping process periodically, every 2 hours or so, until all of the syrup is scraped into the consistency of water ice. Serve after 8 hours.
Arrange the oysters on a bed of ice. Top each oyster with 1/8 teaspoon of wasabi and 1/8 teaspoon caviar. Serve with yuzu granitee.