Hmm.. I never tried it, but I made vegan sushi today actually (see my blog below) as for what can take the place of spicy tuna.. what if you make a mixture of wasabi, water, vinegar, and some sugar and use that to pickle some radish? Or, add a touch of soy sauce and use that to boil the radish quickly for about 5 minutes. Radish has a really nice bite to it, and the wasabi will only kick it up a notch, to be tempered by the sugar and soy sauce. You really want to either boil or pickle it to get rid of the crunch though.
Tell me how it goes: I've never personally tried this at all.
You could also marinade some firm tofu with a mixture like what I suggested to pickle the radish in and roast it in the oven.