Add a hook or drupal_alter in mathjax_init() to let other module easily extend the active status.

function mathjax_init() {
  global $base_path;
  
  $active = mathjax_active() && variable_get('mathjax_enabled', TRUE);
  drupal_alter('mathjax_active', $active);

  // Load Mathjax only on specified nodes
  if ($active) {...}
}