Problem using includes and constants in XML gui

I'm playing with writing an add-on that uses XML files for the GUI. The problem I'm having is using includes and constants. For example, I defined a constant representing a color value (ARGB) to use with the <texturediffuse> tag. When I use the constant I don't see the texture. However, if I use the literal value for the tag, the texture is visible.
The constant definition:
<constant name='mycolor'>FFAA00AA</constant>
Anyone have