Detect an empty theme region in custom panel layout

Greetings:

I have a custom panel layout that is responsive, and it nicely adjusts based on whether there is any panels content within a given region.

However, this layout is being rendered inside of my regular theme, with a standard sidebar_left region that displays regular old Drupal blocks.

I am trying to find the right preprocess function that will allow me to let my panels layout know whether sidebar_left is empty, so that I can adjust my grid and go to 100% width. I've tried all of the obvious candidates (I think) -- for example:

function omega_eps_preprocess_page(&$variables) {
  if (empty($variables['page']['sidebar_first'])) {
    $variables['sb'] = 'no sidebar';
  } else {
    $variables['sb'] = 'sidebar';
  }
}

but I can't find a way to pass this on to eps-grid.tpl.php (which is my custom panel layout living within my theme).

Any pointers would be much appreciated as to whether this is even possible, and if so, where I should be looking.

Thanks!