Applying a (non-multi) ValueConverter the output of a MultiBinding + StringFormat - wpf

Question

Is there a way to apply a (single, not multi) ValueConverter to the output of a MultiBinding which uses StringFormat (i.e. after the string has been formatted).
It would be the equivalent of that code, in which I used an intermediary collapsed TextBlock to do the trick :
<StackPanel>
<TextBox x:Name="textBox1">TB1</TextBox>
<TextBox x:Name="textBox2">TB2</TextBox>
<TextBlock x:Name="textBlock" Visibility="Collapsed">
<TextBlock.Text>
<MultiBinding StringFormat="{}{0}{1}">
<Binding ElementName="textBox1" Path="Text"/>
<Binding ElementName="textBox2" Path="Text"/>
</MultiBinding>
</TextBlock.Text>
</TextBlock>
<TextBlock Text="{Binding ElementName=textBlock,
Path=Text, Converter={StaticResource SingleValueConverter}}" />
</StackPanel>

Answer 1

Here is a hack that does what you want:
public static class Proxy
{
public static readonly DependencyProperty TextProperty = DependencyProperty.RegisterAttached(
"Text",
typeof(string),
typeof(Proxy),
new PropertyMetadata(string.Empty));
public static void SetText(this TextBlock element, string value)
{
element.SetValue(TextProperty, value);
}
[AttachedPropertyBrowsableForChildren(IncludeDescendants = false)]
[AttachedPropertyBrowsableForType(typeof(TextBlock))]
public static string GetText(this TextBlock element)
{
return (string) element.GetValue(TextProperty);
}
}
<StackPanel>
<TextBox x:Name="textBox1">TB1</TextBox>
<TextBox x:Name="textBox2">TB2</TextBox>
<TextBlock Text="{Binding Path=(local:Proxy.Text),
RelativeSource={RelativeSource Self},
Converter={StaticResource SingleValueConverter}}">
<local:Proxy.Text>
<MultiBinding StringFormat="{}{0}{1}">
<Binding ElementName="textBox1" Path="Text" />
<Binding ElementName="textBox2" Path="Text" />
</MultiBinding>
</local:Proxy.Text>
</TextBlock>
</StackPanel>

Answer 2

If you look at the MultiBinding.Converter Property page on MSDN, you will see that you can provide a Converter for a MultiBinding. However, it is not a normal IValueConverter, instead it requires an IMultiValueConverter. It can be used like this:
<TextBlock x:Name="textBlock" Visibility="Collapsed">
<TextBlock.Text>
<MultiBinding StringFormat="{}{0}{1}" Converter="{StaticResource Converter}"
ConverterParameter="SomeValue">
<Binding ElementName="textBox1" Path="Text"/>
<Binding ElementName="textBox2" Path="Text"/>
</MultiBinding>
</TextBlock.Text>
</TextBlock>
An example of an IMultiValueConverter implementation can be found in the linked pages.