Insert/sticky specific post into Loop at specific location

I’m having a hard time determining the first direction to take with this. I’m trying to write a plugin that allows users to pick a location for a post to be inserted into the homepage. For example a user can ‘sticky’ an old post to the 2nd location on their front page. My problem is I don’t know how to insert a post into the loop at a specific point. I’m thinking it should be something like the following (but clearly I’m missing the insert text):

$current_position = $_POST('current_postition');
add_action ('pre_get_posts', 'add_post_to_frontpage');
function add_post_to_frontpage ($query) {
  if ( $query->is_home() && $query->is_main_query() && $query->current_position = $positionlocation ) { 
        //insert a given post into this current_position
     $query->set( 'p=$desired_post');
  }
}

Thanks

EDIT: I added in what I think is a logical way to set the post, but think that this will cause trouble with the posts that follow the post I’m inserting

This took a bit of trial and error but I think I got it. I was getting an infinite loop until I added suppress_filters. After that it was short work.

function insert_post_wpse_96347($posts) {
  global $wp_query;
  $desired_post = 151;
  if (is_main_query() && is_home() &&  0 == get_query_var('paged')) {
    $p2insert = new WP_Query(array('p'=>$desired_post,'suppress_filters'=>true));
    $insert_at = 3;
    if (!empty($p2insert->posts)) {
      array_splice($posts,$insert_at,0,$p2insert->posts);
    }
  }
  return $posts;
}
add_filter('posts_results','insert_post_wpse_96347');

This will force a post, which I have hard-coded, into position 3, again hard-coded. I don’t know how you are saving, or plan to save, your configuration values but that is is the basic “insert” function. This will increase the post count for the first page above the configured value but otherwise preserves normal pagination as far as I can tell. It is tested but barely.