I didn't manage to get this to work properly, so instead I just went with the quick and dirty workaround:
<richtextbox isreadonly="True">
<paragraph>
<run>Please </run>
<hyperlink click="Hyperlink_Click"> click here </hyperlink>
<run> to see more</run>
</paragraph>
</richtextbox>
And then navigate in the HyperLink_Click event
private void Hyperlink_Click(object sender, RoutedEventArgs e)
{
TargetName="ContentFrame"
var uri = new Uri("/Chapters/Pages", UriKind.Relative);
NavigationService.Navigate(uri);
}