Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 1899)
+++ wp-includes/theme.php	(working copy)
@@ -348,6 +348,7 @@
 		}
 
 		$template = trim($template);
+		$theme_root = apply_filters( 'parent_theme_root_' . $template, $theme_root );
 
 		if ( !file_exists("$theme_root/$template/index.php") ) {
 			$parent_dir = dirname(dirname($theme_file));