Making this change is a bit more extensive that updating the css involved. You would essentially need to create or modify the existing js function to show or hide the caption. Once the function call was built you would then attach it to the onClick for your button that you are adding.
If you are comfortable with js scripting and can easily read the gallery js file then you would probably be able to accomplish this, otherwise this may be a difficult task. We cannot offer support for customizing the js code in the gallery, it was only designed to work in it's current state.