[Zope] How do I sort on a dynamicly generated field?

[Jay, Dylan]

I'm trying to get something like this going.

<!--#in qryAQuery sort=doExternalMethod-->
 <tr>
  <td><!--#var doExternalMethod--></td>
  <td><!--#var AnotherField--></td>
 </tr>
<!--#/in-->

I'm sure I read somewhere you can sort on more than just the fields
available in a table, ie calculated data etc but I can't seem to be able to
do it. Any ideas?