`wp-config.php` file to sit above a pristine WordPress directory, whereby the site can symlink their WP directory to a common one, and this file will make sure their `wp-config.php` is the one that gets called. Untested in production. Just an idea right now.

wp-config.php

<?php
$path = str_replace( $_SERVER['DOCUMENT_ROOT'], '', dirname( $_SERVER['SCRIPT_FILENAME'] ) );
$path_parts = explode( '/', $path );
while ( count( $path_parts ) > 0 ) {
	$path = $_SERVER['DOCUMENT_ROOT'] . implode( '/', $path_parts ) . '/wp-config.php';
	if ( file_exists( $path ) ) {
		include( $path );
		break;
	} else {
		array_pop( $path_parts );
	}
}