Hello,
is there anything new about this feature?
		
	 
	
	
	
 
		
			
	
	
		
		Comment in bp-messages-loader.php
/**
 * Implementation of BP_Component for the Messages component.
 *
 * @since BuddyPress (1.5.0)
 */
class BP_Messages_Component extends BP_Component {
	/**
	 * If this is true, the Message autocomplete will return friends only, unless
	 * this is set to false, in which any matching users will be returned.
	 *
	 * @since BuddyPress (1.5)
	 * @var bool
	 */
	public $autocomplete_all;
So yes it is implemented since BP 1.5
By default you get only suggestion of your friends names. You can get all user names, by adding this to your bp-cutom.php file
define( 'BP_MESSAGES_AUTOCOMPLETE_ALL', true );
But never use this constant if you have hundreds of user ! (imagine the length of such a list !)