Hello Mike,
sorry for a late answer, - I missed that Discussion.
What I can say is - the GetMapiProperty works only for simple MAPI properties, not for Multivalued or stream e.g. RTFText.
If you want to access those properties you must go deeper into ExtendedMAPI and use a Mixed (Managed/Native) C++ Library.
From there it's easy to access any Kind of Extended MAPI.
I recommend the Book: Inside MAPI and here is my Samplesolution to get the Job done:
http://www.codeproject.com/KB/office/BridgingTheGap.aspx
Hope this helps, greets - Helmut