One way to bowhunt rabbits with a lab is this: teach the dog to heel - a REALLY solid heel. Build up to incredible levels of temptation, so that the dog will not break heel to chase a rabbit that runs out right in front of him. LOL with your dog, this might be easy. I've been working with my mom's 9-years-old-yet-hyper nutty lab, and it is really difficult. Teach the dog to sit automatically when you stop also.
Anyway, the idea is that you stalk the rabbits (cottontails) with the dog at heel, just like how you would bowhunt rabbits without a dog. You need the dog to stay close and be quiet. No whining or jingling tags on the collar. As soon as you shoot a rabbit, and are sure you made contact, you release the dog with Fetch! and the lab gets to do what it does best. Very exciting retrieve for the dog, and you're not likely to lose the bunny if the shot is less than stellar. Of course, you can do this with probably any dog with enough obedience training. I am trying with my mom's lab instead of my Clumber spaniel because I don't want to hamper my spaniel's natural quartering / hunting style. The lab has little obedience training, and really needs some heeling training to rein her in. My spaniel, on the other hand, needs to range a little more if anything.
Running rabbits doesn't seem to work here, because they always find a pile of rocks to hide in. And if you don't make a good shot, they can disappear down one of these rocky holes before you can grab 'em. That's why I'm working with the lab to retrieve the bunnies, but for bowhunting I don't want her to be hunting them up because she's too fast.