Site injections do no work. As soon as you inject a substance into the body, it is released into the bloodstream and transported throughout the body to form it's mechanism of action.
Site injecting muscles will cause a build up of scar tissue in the area which will make the muscle look bigger, but no localized growth will occur.
You can do a sub q injection anywhere you can pinch a layer of skin with your finger and thumb. You could do it on your chest if you wanted but it would be much easier to do around your belly button.