Really that's intresting, I wonder how many unscrupulous bosses claim that back and either ding spend that much or nothing at all. Any decent accountant must know that then I'm guessing.
In fairness sneaking in a your meal for free would actually upset me a little if I found out, unless you were forthright from the start, then I could actually make that decision when you send your RSVP.
However I would not send an email and threaten to contact HR, that is something you just...