You could mix white and brown rice together which would help hold the onigiri together. What exactly is wrong with the rice? You said it tasted terrible, what exactly is wrong with it? There are a variety of ways to flavor the rice including all kinds of sauces, spices, furikake, and dressings that could either mix in or drip on top as you are eating the onigiri. You could also stuff them to make them more exciting - there are a couple threads on here about fillings. If the texture is not as nice as you'd like, wrap the warm onigiri in saranwrap when you make them, you can let them cool toroom temp before you put them in your lunch. At lunch time, unwrap them and they'll have stayed more moist and stuck together.
There is some soy sauce out there that is gluten-free (my aunt is gluten free and loves Asian foods of all kinds and couldn't give up the soy sauce!).
Here is a list of gluten free soy sauces and stirfry sauces from
http://celiac-disease.com/.