In case anyone needs to send Outlook meeting invitations from C# (or VB.NET, etc.), I have found the following resources useful:
It turned out that adapting none of the code in these links worked correctly (the goal is to have Outlook add the “Attend”, etc. buttons) the only way that worked for me was to send a request, and then cut-n-paste the resulting ICAL file, filling the relevant fields with my data.
It seems Outlook is incredibly finicky in what it accepts, and incredibly liberal in what it generates.