Vista - Trigger on user dependency property

I created a button that can be up, down, or "stuck down" for want of a better
description. I created a new type that overrides Button and created styles
and content templates linked to IsPressed state. I have 3 different
graphical representations of my custom button.

To display the button in the "stuck down" position (ie when used as part of
a RadioGroup) I temporarily used the existing IsEnabled = "true" state to
trigger the display of the "stuck down" position (ande set it from code when
appropriate). But that was only because I was in a hurry and I couldn't
figure out how to do anything more appropriate.
It all works, but clearly this is not what "IsEnabled" is for. What is
more, should anyone try to disable my button they'll get some unintended
behaviour.

Now, revisiting this with a bit more breathing time, I'd like to use a
custom dependency property (ie "IsStuckDown") that will be set from code and
will be referenced in the Style.Triggers as below

<Trigger Property="Button.IsStuckDown" Value="true">
<Setter Property="Template" Value="{StaticResource
ButtonStuckDownTemplate}" />


This is all fine except that when I add
="Button.IsStuckDown"
in place of
="Button.IsEnabled"
in the code above
I get a BAML runtime exception that tells me very little.

the dependency property "IsStuckDown" is all there and working.

Is this supposed to be possible ? Or am I doing something that is not
actually supported ?


Thanks
--
Griff
(trying to make an industrial UI with XAML/WPF/c#)