You can also add HTML on *all* pages with the pagetop/foot directives -- I use this to add a global 'University of Auckland' footer on all our pages.
The charset directive affects the HTML headers, you should get a different MIME encoding header if you use it. However it indeed seems to to be setting the XML header correctly for some reason. This may be fixed in v2.15b3, although it is hard for me to test here
. Email me directly if you are being sufficiently inconvenienced by this that you would like a copy of the beta to try out.