I'm thinking just an email, saying, "Hi, we're finalizing the meals for the hotel. Are you able to join us next month?" something like that?
edit - our RSVP date is today, wedding is Aug 3, but our local caligrapher needs to get started on the seating cards stat, because they're a bit involved. So I was going to give them a few days after the RSVP date and then contact them.