What I am trying to do is display Google AdSense ads (or other ads) on my website and others, in a rotation with other ads. I know this is somehow possible (without violating Google's rules) b/c the PHP program OpenX ad server does it.
It is possible with server side scripts - for example the server could keep a list of five ad snippets, and randomly deliver one of them on each request. That way the client always sees the unmodified HTML+JS that Google gave you.
It's also a bit simpler to implement, because it doesn't imply parsing HTML.