YouTube uses NDK, most developers do not have access to this.
official customizing of the default player is limited to color and bitmap changes.
see UI Fileds -> trickplaybar
More can be done if you walk through the video player UI node and find each element, but I don't know if it can be guaranteed that the elements will be consistently be where you find them. Roku could make a changes that could break at any time with a future update.
You could build a custom video player UI but that would be a very involved project.