Customizing a plugin function using a hook

Maybe try the following:

function set_robotsmeta_default( $post_object ) {
    if (!isset($post_object->robotsmeta) || $post_object->robotsmeta == "") {
        $post_object->robotsmeta = "noindex,nofollow";
    }
    return $post_object;
}
add_action( 'the_post', 'set_robotsmeta_default' );

EDIT: Since the above didn’t work, the code below may work, by editing the global $post object before the plugin add_meta_boxes is called. I just don’t know if this will affect posts that already have that value set.

function set_robotsmeta_default() {
    global $post;
    if (!isset($post->robotsmeta) || $post->robotsmeta == "") {
        $post->robotsmeta = "noindex,nofollow";
    }  
}
add_action( 'add_meta_boxes', 'set_robotsmeta_default', 1);