My organization also has a policy as you describe. Everything *.ourdomain.com is treated like an "Intranet" (even though the same domain with www. is used for a public-facing website) and IE 10 has, by default, the "Display intranet sites in Compatibility Mode" checked.
I believe up through IE 9, the group policy overrode the meta tag for sure.
IE 10 seems to behave differently. As long as the X-UA-Compatible <meta>
tag is at, or very near, the opening tag, then IE 10 will respect the meta tag above the group policy.
I have just now tested with the HTTP header by adding it to an .htaccess. To my surprise, not only did IE 10 respect it, but also IE 9!
I don't know enough about Microsoft group policies to tell you it will absolutely work in your situation.