Currently one of the ways to do that would be to do something like http://doc.silverstripe.org/doku.php?id=recipes:customising-content-in-your-templates. In the upcoming 2.4 release, it has a neat little tokenized system for inserting snippets into the content area which is a bit nicer then a str_replace.
Really old post (I know) but the link is dead and the question is not answered anywhere else. I'm trying to embed html using the insert html function, like this:
<div id="surveyMonkeyInfo"><div><script src="https://www.surveymonkey.com/jsEmbed.aspx?sm=xxxxxxxxxxxxxx"> </script></div>Create your free online surveys with <a href="https://www.surveymonkey.com/">SurveyMonkey</a> , the world's leading questionnaire tool.</div>
This is SurveyMonkey's latest embed functionality. Hopefully Silverstripe can support this.