Not sure what you mean by "lock the focus on". AI Servo will track focus on a moving subject. Is that "locking on"? Or do you mean pre-focus, as in set the focus and then not change it? Using C. Fn 4, you can set your focus with the back * button, then concentrate on framing the image and nail your timing to trigger the shutter at exactly the proper moment.
Another opinion regarding shooting wide open (there are a couple threads on this topic on another sports board right now). While I shoot my f/2.8 lenses at f/2.8 most of the time, it's worth noting that shooting stopped down a little will increase the sharpness of even the best of lenses, with the added benefit of marginally increasing DOF. Of course, you can only do this when there is enough light to allow for stopping down while maintaining adequate shutter speeds.
For example, if you stop down 2 stops, from f/2.8 to f/5.6 it only increases the DOF by 100% (or twice the DOF). That's a little counter intuitive since moving from f/2.8 to f/5.6 is an aperture change of 2 stops, and 2 stops means 4X less light into the camera, but in the case of Depth of field, the math is slightly different and it only increase DOF by a factor of 2.
If your backgrounds are clean, and there is a lot of "air" behind your subjects (relatively large distance between the subject and items in the background), your "bokeh" will still be pretty clean, and you'll have the added benefit of increased sharpness. In addition, many plays ini softball and baseball (and quite a few other field sports) involve 2 or more players, and increasing DOF makes the chances higher that you'll get all the players in focus.
