Well .. you create your base control
public abstract class BaseUserControl : UserControl{...}
then in the XAML file :
<Controls:BaseUserControl x:Class="Termo.Win.Controls.ChildControl"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:Controls="clr-namespace:Namespace.Of.Your.BaseControl">
And that should work.
EDIT: Hmm.. this example is useful when you have a base control without XAML and then inherit from it. The other way around(from a base control with Xaml) – I’m not sure how you can go about it.
EDIT2: Apparently from this post + comments i take that what you want might not be possible.