A: 

Ended up finding the configuration option after perusing the XSD in the source.

The relevant attribute is allowVanityHeader.

<managedFusion.rewriter xmlns="http://managedfusion.com/xsd/managedFusion/rewriter"&gt;
    <rewriter allowVanityHeader="false" />
</managedFusion.rewriter>

(Included in the <configuration> node of web.config.)

josh3736