i know that there is a canna whose root is used to make 'arrowroot flour.' i've never heard of cannas being toxic, and I would assume that as long as the cannas were grown in herb-/pest-/insecticide free place it would be safe to serve food on them.

I did a quick search online and found some sites stating that parts of the plant are poisonous if not all. Honestly, if you don't find the plants on any lists of edible plants, then it's best to just assume it is poisonous.
But, that doesn't mean you couldn't serve food on it. I would maybe call some local nurseries and ask them what they think. Some plants are only harmful if bitten into. If you are just going to be laying bread or some other food that can just be easily picked up off the leave with out puncturing the leave, then you may be just fine.
Some plants though, can have an oily like substance on the outside that can be poisonous. I doubt canna's are like that because if they were, not as many people would grow them because they'd have to be super careful each time the worked in the garden.
If you do use them, just play it safe and make sure whatever is laid on them is a finger food that a utensil will not be needed because a utensil will puncture the leaves far too easily (even a spoon could scrap into the leaf's flesh.

Deer don't eat cannas in my yard, and they eat almost everything else. The other plants that are similarly unmolested are often quite toxic.

great feedback, thank you everyone!

the darling deer have kept the canna's in my friends yard trimmed to less than a foot tall all summer---even with deer away spray'd on them! yes, virgina, the deer do eat canna!