Thank you for that idea. I don’t know how or whether to implement it. Here is my code:
add_action('genesis_before_sidebar_widget_area', 'sidebar_custom_field');
function sidebar_custom_field() {
if ( is_single() && genesis_get_custom_field('sidebar_custom_field') )
echo '<div class="widget">'. genesis_get_custom_field('sidebar_custom_field') .'</div>';
}