I'm trying to modify the user dropdown in the post edit screen to show only (a) the current user and (b) the current author of the post. I can't do this using wp_dropdown_users_args because the meta box in the block editor gets the list of users via a REST query. (wp_dropdown_users_args works fine for the user dropdown in the "Quick Edit" form, however.)

rest_user_query can modify this dropdown for me, but I can't seem to get any info about the post I'm editing, so I'm unable to determine who the current author is. I can add the current user to the include value, but then the dropdown doesn't appear at all (I assume because it has only returned a single user).

What I want to do is something like this, but I don't know how to define the $POST_AUTHOR variable here:

function rest_user_filter( $prepared_args, $request = NULL ) {
  $args['include'] = [ get_current_user_id() ];
  array_push( $args['include'], $POST_AUTHOR;
  return $args;
}
add_filter( 'rest_user_query', 'rest_user_filter' );

I'd also only like to do this on the post edit screen, of course, too.