do_action and errors

Hi,
I’m trying to write an plugin which does some processing during the register_post hook, and which returns some errors back to the core register_new_user() function in wp-login.php

register_new_user() does this:

do_action('register_post', $user_login, $user_email, $errors);

$errors = apply_filters( 'registration_errors', $errors );

if ( $errors->get_error_code() )
	return $errors;

But in my plugin, I can’t find any way to get a reference to the $errors, add new ones, and pass it back to register_new_user()

I’ve tried all sorts of methods (and probably missed the very simple one that I need).

Can anyone help please?

Regards,
Andy