can i execute custom php code as a rule action?

i used to do this in drupal 7, but i cannot see how to do this in backdrop. is there a way to execute a custom php function as the action for a rule?

alternatively, is there a reliable way to hook into the normal process flow and execute some custom code AFTER a node has been saved to the database? hook_insert() etc all seem to happen prior to the node data being saved in the database, and therefore there is no nid yet for the new node. have i missed something there?

thanks for any ideas.