I started fighting with this problem last evening and (wouldn't you know it) found the answer as soon as I posted.
You can control header-content injection in the JBoss specific portlet descriptor (though I hadn't found it because I've been developing portable portlets).
Is there some way to access/change these settings programmatically?
I was afraid of that ... maybe there will be a standardized mechanism in the next Portlet specification. I wanted to change renderers based on user privilege, but will have to look at another mechanism.
Thanks for the help Roy!