How to use shortcode attribute in separate function

I’m working on a WordPress site and have installed a slider plugin that registers a slider on the backend (or multiple sliders) and then displays it on the front end with a couple of simple php snippets. I’m trying to modify the plugin so I can do the same thing with a shortcode. I want the shortcode to be able to register the plugin on the backend and display it on the front end using something like: [responsive_gallery slider_name="slider"].

I have that part down, and I can even display the slider using the shortcode. I’m just having trouble registering it on the backend. My code at the bottom works, but I was hoping there was a way to take the value stored in the shortcode attributes array ($slider_name) and use it in the set_flexslider_hg_rotators() function. I’ve been trying for hours and can’t get anything to work. Any suggestions on how to best do this?

Here is the plugin code:

Backend Registration:

function set_flexslider_hg_rotators()
{
  $rotators = array();
  $rotators['homepage'] = array( 'size' => 'homepage-rotator' );
  return $rotators;
}
add_filter('flexslider_hg_rotators', 'set_flexslider_hg_rotators');

Front End Display:

<?php if(function_exists('show_flexslider_rotator')) echo show_flexslider_rotator( 'homepage' ); ?>

Here is my working code:

function responsive_gallery_shortcode($atts, $content=null) {
    extract(shortcode_atts( array('slider_name' => 'product_page') , $atts));

    if(function_exists('show_flexslider_rotator')) echo show_flexslider_rotator( $slider_name );
    add_image_size( $slider_name , '550', '250', true );
}
add_shortcode('responsive_gallery', 'responsive_gallery_shortcode');        

function set_flexslider_hg_rotators() {
    $rotators = array();
    $rotators[ 'product_page' ] = array( 'size' => 'product_page' );
    return $rotators;
}   
add_filter('flexslider_hg_rotators', 'set_flexslider_hg_rotators');

I want to be able to use $slider_name in the set_flexslider_hg_rotators function, but am unsure of what would be the best way to do that (or if it’s even possible). Would like it to look something like:

function set_flexslider_hg_rotators()
{
  $rotators = array();
  $rotators[$slider_name] = array( 'size' => $slider_name );
  return $rotators;
}
add_filter('flexslider_hg_rotators', 'set_flexslider_hg_rotators');

Since $slider_name is not global, nothing is returned from this function (as expected). Is there a way to so something similar?