Hello David Brenchley,
The proper way to do this would be to establish another binding between a TxtDataProvider and the summary_unformatted property with OneWayToSource mode, but since this is missing from Silverlight, unfortunately, the only way to make this work reliably is to subscribe to the DocumentContentChanged event of the RadRichTextBox and using TxtFormatProvider, in code behind, update the desired property.
All the best,
Ivailo
the Telerik team