Vista - Custom DropDownButton control with DropDownArrowTemplate?

I am using WinFX beta 2 on WinXP sp2 cz.

I am creating DropDownButton control derived from ToggleButton (it uses
Popup).
It changes its VisualTree when loaded so that ContentPresenter is put to new
DockPanel together with DropDownArrow which is created from ControlTemplate
specified in DropDownArrowTemplate dependency property.
It works, but I want to bind Fill property of Polygon (triangle) in my
default DropDownArrowTemplate to DropDownButton.BorderBrush. How to do it?
Nor TemplateBinding neither RelativeSource with TemplatedParent works.
DropDownArrow is created from DropDownArrowTemplate by calling
this.DropDownArrowTemplate.LoadContent() as FrameworkElement.
Is it OK? Maybe TemplatedParent of DropDownArrowTemplate is null because it
is added to DropDownButton after LoadContent call.