I’ve written a VSTO add-in sample that implements a form region with a rich text control that allows capturing rich text notes and additional information for appointments and meetings.
How it works
The add-in implements the form region displayed in the screenshot below for compose-mode meetings. Upon saving the appointment or meeting invite the content of the rich text editor is saved in a named property called additionalRTFNotes.
The property definition is http://schemas.microsoft.com/mapi/string/{00020329-0000-0000-C000-000000000046}/additionalRTFNotes and the property namespace is PS_PUBLIC_STRINGS.
For read-mode meetings (in the calendar view reading pane), the add-in implements a read-mode form region that only displays a read-only (non-editable) rich text editor.
Resources
The installers as well as the source code are available for download. The selection change logic still needs some polishing to update the controls accordingly, like for example enable the bold, italic and underline toggle buttons to match the style of the selected text but other than that I think it does the trick.
Lastly
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NB: If you run into any problems, please let me know and I will try and address them as soon as I can.