Call MarkupExtension constructor with parameters from datacontext.

I've created a markupextenstion that will return the properties of a given type, however I don't know how to instantiate it in XAML, because I want it to use a property value rather than a static type:

Public Class PropertyEnumerationExtension
        Inherits Markup.MarkupExtension

        Public Property ObjectType() As Type

        Public Sub New(objectType As Type)
            Me.ObjectType = objectType
        End Sub
    End Class

...and the supposed XAML:

<ComboBox ItemsSource="{Binding Source={my:PropertyEnumeration {Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type editor:GroupExpression}}, Path=DataContext.ReflectedType}}}" />

I know that my problem is that bindings are used on dependencyproperties, but there must be another way to reference a runtime property?