Add content to the top of each sidebar on the Widgets admin screen.

gistfile1.aw

<?php

add_action( 'dynamic_sidebar', 'gmj_add_content_to_top_of_sidebars_on_widgets_screen' );
/**
 * Add content to the top of each sidebar on the Widgets admin screen.
 *
 * @author Gary Jones
 *
 * @global array $wp_registered_widgets All registered widgets
 * @param  array $current_widget        The current widget about to have its form shown
 * @return null                         Returns null if not admin, or not the first widget in the sidebar
 */
function gmj_add_content_to_top_of_sidebars_on_widgets_screen( array $current_widget ) {

	global $wp_registered_widgets;

	// Only run on the Widgets admin screen, not the front-end
	if ( ! is_admin() )
		return;

	// Get all sidebars and their widgets
	$sidebars_widgets = wp_get_sidebars_widgets();

	// Optionally remove looping through Inactive Widgets
	unset( $sidebars_widgets['wp_inactive_widgets'] );

	// Get current sidebar ID
	foreach( $sidebars_widgets as $sidebars => $widgets ){
		for( $i = 0; $i < count( $widgets ); $i++ ) {
			if ( $current_widget['id'] === $widgets[$i]) {
				$current_sidebar_id = $sidebars;
				break 2;
			}
		}
	}

	// Bail if sidebar not found (e.g. Inactive Widgets, which we unset earlier)
	if ( ! isset( $current_sidebar_id ) )
		return;

	// Get first widget ID in the current sidebar
	foreach( $sidebars_widgets[$current_sidebar_id] as $key => $value ) {
		$first_widget_id = $value;
		break;
	}

	// Bail if we're not about to show the first widget form
	if ( $first_widget_id !== $current_widget['id'] )
		return;

	// Now echo something awesome at the top of each sidebar!
	echo '<p class="sidebar-awesomeness">Foobar for ' . $current_sidebar_id . '!</p>';

}