Basically there's a CDO property (0x8002) which controls whether
outlook should render the preview pane or not. When sending the message
with the custom code, if you set this to zero, when received your mail
will render in the preview pane correctly.

It was a bit tricky to get working, in the end I had to write an addin
to watch for send events with the message class matching my own and
then do the following (in the current mail send event):

If objCurMail.MessageClass = "IPM.Note.MyMessageClass" Then
        On Error Resume Next
        Dim objCDOMail As MAPI.Message

        objCurMail.Save 'Need to save to get an entryID

        Dim strEntryID
        Dim strStoreID
        strEntryID = objCurMail.EntryID
        strStoreID = objCurMail.Parent.StoreID
        'Get the mail via CDO
        Set objCDOMail = gobjCDO.Session.GetMessage(strEntryID,
strStoreID)

        If Not objCDOMail Is Nothing Then
            'We've got our mail, set the property
            objCDOMail.Fields.Add -2147352573, 0, 0,
"2903020000000000C000000000000046"
            'Update the mail
            objCDOMail.Update True
        End If

        'Send it via CDO
        objCDOMail.Send
        'Use our addin to close the mail and the inspector
        objCurMail.Close (olSave)
        objCurInsp.Close (olSave)

        Set objCDOMail = Nothing
        Set objCurMail = Nothing
End If

Now I'm no Outlook programming guru, so this can probably be tidied up
lots and tested further, but now when I send an email, it arrives and
the preview pane works - and I need to get the rest of the development
finished now!

Feel free to email if you have any comments/suggestions, I hope this is
helpful to someone!