This may work, but there are some things you might want to consider. I'm going to assume it's a 2o2o (two on, two off) behavior you're asking for.

Depending on what kind of behavior you expect from the dog's performance, a toy could end up shifting the dog's attention to the ball, instead of the behavior. For example, what will happen if the ball isn't there? Will your dog still perform a 2o2o every time with or without the ball?

I personally don't like rewarding my dogs with their heads down because I believe it encourages them to come down too hard on their shoulders. By rewarding my dogs with their head up at the end of the contact I feel it encourages them to lean back while descending. Some people reward dogs with their heads down for their own reasons though, that's just my personal opinion. 

I teach a 2o2o behavior by rewarding the dog for finding a different surface, and placing their rear feet half on one surface, half on another. I like to use stairs, curbs, wood and grass, whatever I can find. I then move to the bottom of the contact when I know my dog will offer a 2o2o behavior off of contacts first.  

I then start placing the dog higher and higher on the contact asking for the same behavior each and every time. I might agree to using a toy at the end once the dog knows their job and needs something to target to, simply to help increase their speed. I personally don't think it's a good idea in the learning stages, however.

If your dog really loves toys consider asking for the behavior and using your toy as a reward for the behavior, not as an aid to get the dog to actually perform the behavior. 

Katie