Quickly editing private varriables

In my project I want quickly read from and make modifications to a private-ish variable. I was initially going to do reflection, but my thinking was that it would be too inefficient.

public class SomeControllerInALibraryThatICantEdit {

    //I can easily get an instance of the controller
    public static final SomeControllerInLargeLibrary instance;

    //I need to read from and make changes to the data object quickly.
    //It is final, so I don't need to worry about this object changing or being reset.
    private final Subclass dataClass = new Subclass();

    private class Subclass {

        //I can get this once then treat it like any other array.
        public float[] arr;


        //This is the problematic part. I need to be able to quickly check changes and 
        //set this every update. Keeping in mind there are multiple dataPoints.
        public short dataPoint;

    }

}

What is a quick and efficient way to get and set dataPoint? I can use reflection, but I am hoping there is a better way to approach this problem. Can I in some way make a Pojo that represents Subclass and link their pointers together?

If I just fork the library and make it work better with what I am doing, then that defeats the purpose of my project.