Correct markup for "et al" in biblioref

I'm doing a biblioentry for the Agile Manifesto, which has 17 authors
(at least:).  Rather than type them all in, I'd like to put in just the
first one (alphabetically), followed by "et al."

Right now, my markup looks like this:

  <biblioentry xml:id="AgileManifesto">
    <abbrev>AgileManifesto</abbrev>
    <author>
      <personname>
        <firstname>Kent</firstname>
        <surname>Beck</surname>
      </personname>
    </author>
    <author>
      <personname>et al</personname>
    </author>
    <title>The Agile Manifesto</title>
    <bibliosource class="uri">
      <link
xl:href="http://agilemanifesto.org">http://agilemanifesto.org</link>
    </bibliosource>
  </biblioentry>


That renders acceptably, but does seem to be at least mild tag abuse.

Can anyone suggest a better way to mark this up?

Thanks,
Dick Hamilton
http://rlhamilton.net