It turns out you can add things to the header with this:
<% Response.AddHeader "CustomHeader","CustomValue" %>
It also appears that it doesn't matter where I put it on the page, so I can stick it in the common include, and all pages will get the custom header. In this case:
<% Response.AddHeader "X-UA-Compatible","IE=Edge,chrome=1" %>
Wish I had known about this way sooner! Here's the spec:
http://msdn.microsoft.com/en-us/library/ms524327(v=vs.90).aspx