What do you mean by effective? All organic treatments that I know of are oils off of plants and trees. If they hit the exoskeleton on application they will dissolve it, but if they don't hit it will/may only deter them.
Take marginal effectiveness and the neurotic nature of those who want organic pest control and expect retreats as they don't want to see any insects. Charge accordingly for retreats.
a 5 or 6 foot black snake works pretty good. We have a couple in the around the barn and seldom ever see signs of a mouse or rat or even a chipmunk. I imagine your customers wouldn't be real please with you dropping off snakes to control their rodent problems though!
Hardware cloth would be organic. I've read rat teeth are harder than steel, but they don't seem to want to wear them down chewing wire.
That's good answer- I'm thinking about raising certified organic chickens and just trying to brainstorm some ideas. I help raise conventional turkeys at this point using toxic rat bait for rodent control.