Include post id[s] into WP_Query()

I think you should be able to do this with the user_has_cap filter.

Because that filter passes through the post ID in the $args for the edit_post capability, you can use that to check if the current user has been artificially given this capability – and if so, force the edit_post capability to be true for that instance.

Building from the example in the docs linked above, you’d be looking at something like this:

add_filter( 'user_has_cap', 'wpse_227418_edit_extra_posts', 10, 3 );

function wpse_227418_edit_extra_posts($allcaps, $cap, $args){

  if($args[0] != "edit_post"){
    return $allcaps; // bail out, we're not talking about editing a post here
  }

  if(get_post_meta($args[2], "additional_author", true) == $args[1]){
    // get the meta key 'additional author', checking that it matches the User ID we're looking at the capabilities for
    $allcaps[$cap[0]] = true; // make sure edit_posts is true for this author and this post ID
  }

  return $allcaps;

}

This is pretty quickly put together and is untested so you’d certainly want to test and modify to your needs, but I think this will get you what you want. Of course, adjust the meta_key referred to above to match the key you’re after.

References:

• user_has_cap filter on Codex
• has_cap() function in WP_User class (the function that calls the above filter)