I'm running the latest version (10.10) of S2 and try to add custom fields (from the Advanced Custom Fields plugin) to the mail body.
I'm using the s2_html_email and s2_plain_email filters (S2-API) and a custom function to modify the mail body. This works if the post is saved before it's published. If it's published directly without have been saved as draft the get_the_ID() and equivalents are still empty and I can't post the meta data with
get_post_meta(get_the_ID(),'<advanced custom field>',true);