[racket] Injecting HTML into a scribble document?

I'd like to be able to inject some HTML fragments into a Scribble
document, so that when the Scribble documentation renders to html,
those fragments are carried along.

  * For example, if I have a Google Analytics snippet that needs to be
injected within the <head/> of the document, I'd like to have that
done at the documentation generation time, rather than as a separate
step.

  * As another hypothetical scenario, I might want to add an IFRAME to
another html site, embedded within the flow of my document.


Both of these are very specific to HTML rendering, so they probably
wouldn't be relevant if one is Scribbling to the other formats (PDF,
text)


Do such mechanisms already exist in Scribble?  If so, what should I be
looking at?