I've discovered the secret to making sushi with leafy vegetables as a wrap instead of sushi nori - wilt the leaves first! If the leaves are still fresh and crunchy, they don't like to stick to each other or the rice. But when you wilt them slightly, then lay them flat on top of each other, they make a pretty nice sheet for sushi. If you're going to try this, you'll need either large spinach leaves or to lay the leaves several deep and wide - you want it wide enough to wrap all the way around your filling.
A yummy alternative if you don't have nori on hand! Enjoy!