Setting header Content-type and bloginfo(‘html_type’)

So, being the greedy person I am, I’ve been trying to get
Wordpress to support a XHTML+MathML+SVG DOCTYPE, set my mimetype to application/xhtml+xml and so forth.

I don’t mind setting *most* of this in the theme… At least
the <?xml …> tags and so forth probably ought to be there,
I’d prefer to know how bloginfo derives html_type and where
the php header is generated so I could write a plugin to make that configurable from the admin interface… Any info is appreciated, otherwise I’ll keep digging through the code to find the best place to modify this…

Yes, I know this will break IE. If you know of a cleaner, easier way to take a WP theme and modify it (and WP) to display MathML and SVG within posts, I’d be grateful.

I appreciate any help anyone can offer.
Thanks much,
John