DesigntimeBinding.cs

using System.ComponentModel;
using System.Windows;
using System.Windows.Data;

public class DesigntimeBinding : Binding
{
    private static readonly DependencyObject _do = new DependencyObject();
    public DesigntimeBinding(string path)
        :base(path)
    {
    }

    public object DesignTimeValue
    {
        set
        {
            if (DesignerProperties.GetIsInDesignMode(_do))
            {
                FallbackValue = value;
            }
        }
    }
}

Sample.xaml

 <Rectangle  Fill="Blue" Width="{local:DesigntimeBinding Meh, DesignTimeValue=200}"/>