Have you tried setting it explicitly in the templates for that queue? In general RT defers to headers present in templates over auto-generated ones.
This likely means duplication of public-facing templates, but that's probably preferable to hacking the codebase to wedge this in. -- Cambridge Energy Alliance: Save money. Save the planet. Discover RT's hidden secrets with RT Essentials from O'Reilly Media. Buy a copy at http://rtbook.bestpractical.com
