XMPP can carry anything, but it can more easily carry XML-based formats like XHTML. But given that, yes. Formatted IM messages are already specified as a subset of XHTML and you can trivially create your own messages that do whatever custom thing you want with XHTML on your page, or float your custom addition on top of existing messages as a namespaced addition.
The most interesting obvious usecase to me would be using XMPP and ejabberd as a message backplane for your website, hanging various components off of ejabberd that can integrate with your web connection.
Because then you'd have to implement your own presence, discovery, and routing mechanisms. You get all of that (and more) for free with an XMPP backplane like ejabberd. Even if you decide it's better to roll your own, you'll then have to come up with a consistency and clustering model in order to scale it.
With the advantages xmpp may or may not have over http/html, this could be interesting.