Hook for modifying book tree?

Would it be possible to add a hook for modifying $tree before node->body is loaded via book_export_traverse() in print.pages.inc.
Something like $tree = module_invoke_all('print_modify_tree', &$tree, $nid); so that tree data could be easily manipulated before book body is loaded. That way exclusion/inclusion of nodes would be easy for third party modules extending your module.

Thanks for your module!