Are just avoiding added fat, or are you also avoiding pork-based sausage?
I would say hawgsquatch got it right on the money. Apple along with some traditional herbs and spices like sage would go great. I've managed perfectly fine chicken sausage with just butter as the added fat, along with onion and apple to bring the moisture and bind it together. Also, I would add that you should avoid processing the meat too much if you're going lean (especially if you will use poultry) - lean sausage that has been overly-ground will end up being spongy, and it is a very unpleasant texture on the tongue.
In venison sausage I have used onions and bacon for some moisture. Also believe it or not grinding in some ice during processing will leave lots of moisture in the sausage, just keep it cold when you handle it.
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