Add default content to post (for specific category)

I’m trying to add some default content to my posts using this code in my functions.php:

add_filter( 'default_content', 'my_editor_content' );
function my_editor_content( $content ) {
$content = "My html content.";
return $content;
}

It works fine, but I would like to add the content to some categories only. This is tricky because the default content gets added while creating a new post, (so it is without a category). I followed this thread:
Force category choice before creating new post?
and the author came up with a way of forcing you to choose a category before wp creates the new post, but I don’t know how to edit the default content code so it is only applies to some categories?

default_content runs when a post is loaded into the editor, actually. You can check for categories when the post is saved, but you’d want the save_post hook, probably. You want to check the $_REQUEST global.

add_filter( 'default_content', 'my_editor_content' );
function my_editor_content( $content ) {
  global $_REQUEST;
  if (isset($_REQUEST['post_category']) && in_array($some_category_id,$_REQUEST['post_category'])) {
    $content = "My html content.";
  }
  return $content;
}

… assuming no syntax errors. 🙂

You probably also want to check if the post_content is empty and insert only it is.