Okay, based on some reading of the RFCs, I think this will capture any meeting-related message from any source (even Google Calendar), with a fairly low chance of capturing anything else.
Code:
If ALL of the following conditions are met:
Body contains Content-Type: text/calendar
Body contains method=
Body contains METHOD:
Body contains VEVENT
Body contains VCALENDAR
Perform the following actions:
File into Folder <folder_name>
I'll test in the morning with the "assistant" using various clients.