I could be wrong, but I do not think there is a non-hackish solution. AFAIK, the default html forms are generated directly from a java class. So there is not template to modify. I believe only format="xml" uses a kind of 'template' in the form of xsl files in the C:\ColdFusion8\wwwroot\CFIDE\scripts\xsl directory.
Assuming you have tried other methods without success, you might be able to trick CF by setting the header inside a custom tag. Not a great solution, but it should work.