Hi John,
Don't do that, the XML is checked for well-formedness and anything not conforming to the HM XML schema will be stripped out silently. Annoying, I know, but XML hast to be 100% syntax-perfect, that's the only way you can guarantee correct translation into so many different output formats.
However, what is supported is personal meta tags that you can add at the top of the topic, inside the <topic> </topic> tag pair, like this:
<meta name="mymetatag" value="My Meta Information" />
You can read the values stored there into your topic body if you want, using the TOPICMETA variable like this:
<%TOPICMETA:MYMETATAG%>
You type this in manually. The variable above would insert the value from the example tag above, if it is present in the topic in which the variable is used.