A new version is now available for download: this one supports the Outlook 2013 preview, and includes a few changes to the handling of reply states of meeting requests.

• Outlook 2013 includes a missing attachment check, so I've disabled it in the add-in.
• Another feature of Outlook 2013 is an "inline response" mechanism - I've added appropriate buttons to the ribbon when one is in progress.
• The previous release included only a no-forward button for meeting requests; this version includes all three, and doesn't let you re-enable them on receipt. (Unlike for email messages - yes, I know it's inconsistent, and I'm not sure which is the better choice.)