I like to substitute the rice with eggs. Take a couple of eggs and whisk them together really well. Get a really good non-stick pan and cook a very thin layer of egg on the surface of the pan. Take the egg that you just cooked and sprinkle a bit of vinegar on it (for flavor). Lay the egg on your nori sheet and add vegetables, fish, etc and roll it up as usual. This is where I got the idea... http://www.marksdailyapple.com/sushi-with-a-twist/#more-10774
You could make those cone shaped hand rolls and skip the rice altogether or if you really want to play with a rice substitute I'd try playing with some kind of cold mash, like maybe mashed parsnip or cauliflower to bind your crunchier cauliflower bits.
Try this http://www.konjacfoods.com/product/15.htm
It's made from konjac, a Japanese plant that is zero carb and zero calorie. I've not tried the orzo rice myself but have been eating konjac noodles for years.
Just add avocado to hold it all together,and do more of a handroll style...raw fish, avo, green onion, roe, homemade fermented ginger...yummy!
I never miss the rice at Japanese restaurants anymore...I get to eat more sashimi, and most places will do a handroll for me with no rice.
If you search for raw diet recipes you will find Cauliflower rice recipe for sushi. I've made it and it's a very good substitute, you just have to watch the salt. The Raw diet has a lot of things that could be paleo (except it's vegan) but to add to paleo…good resource.
I use kelp noodles. They are clear noodle-like pieces of kelp that have no carbs or calories. They add bulk to my salmon/avocado/cucumber rolls and provide a neutral backdrop for the other flavors. You can get them at WHole Foods. I also use them for Pad Thai. They are not sticky like rice, so you have to roll really tightly, but they will work.
i just made maki rolls using crushed chickpeas, flavoured with curry, onion, garlic, tahini, sea salt. basically a lumpy curried hummus. nice and sticky, goes great with any vegetables, and you don't even have to dip it in soy sauce, as it has tons of flavour all on its own.
Do a hand roll like others have suggested. You can fill the inside with more veggies instead of rice - kinda like Thai or Vietnamese spring rolls, but lose the rice paper wrapper, and replace with nori. I've tried hand rolls with ahi, lettuce, cucumber, julienned veggies (bagged broccoli/carrot slaw veggie mix works too), avocado and optional mayo. Drizzle with shoyu (soy sauce). It's not sushi without rice, but there are good alternatives if you're trying to eliminate grains.