how to write a post-save trigger
Just to break the process down a bit (based on your other questions:1,2); when you submit a node form:


node_form_submit() calls node_save().
node_save() immediately calls node_invoke_nodeapi($node, 'presave'); to inform other modules about the presave.
node_save() then sets up a few properties on the node object, and the node record is written to the node table in the database using drupal_write_record().
At the very, very end of the function (after the data has been written to the node table), node_invoke_nodeapi() is called again with either 'insert' or 'update', depending on the current operation being run.


If you look through those functions, and the summary above, you'll see that the node data is 100%, definitely committed to the database before the 'insert' or 'update' operations are run for hook_nodeapi().

If that's not happening for your installation I would strongly recommend replacing core with a fresh version, as it sounds like something's gone horribly wrong!

Getting to the actual question; the only way you can add a post-save hook is to add another line in the core node_save() function to invoke it. But you would be adding it immediately after the call to node_invoke_nodeapi($node, $op); (where $op is 'insert' or 'update') anyway, so I'm not sure that would help.