"Prefix 'x' does not map to a namespace"
Found a way around it: rather than have XamlReader parse a string, it worked better if I gave it an XmlReader. The fragment of XML with the DataTemplate defined in it was part of a larger XML document that had all its namespaces defined in its root. This had already been read into an XDocument, and out of which I'd grabbed the XElement with the ResourceDictionary defined in it. The new code, part of MainWindow.xaml.cs, looks like this:
ResourceDictionary dictionary = XamlReader.Load(myXElement.CreateReader()) as ResourceDictionary;this.Resources.MergedDictionaries.Add(dictionary);This threw a different exception, where it couldn't resolve the type of (http://myschemas/MyProfile)Binding. It turns out that you need to qualify the namespaces of everything, including the {Binding ...} references. So the XML fragment had to be amended to:
<xm:TextBox Text="{xm:Binding Path=MessageID}"/>Now XamlParser knew that Binding was a type in the "http://schemas.microsoft.com..." namespace.