I have found the following works:
<rules>
<logger name="*" minlevel="Info" writeTo="logfile" />
<logger name="*" minlevel="Trace" maxLevel="Debug" writeTo="logfile" >
<filters defaultAction="Ignore">
<when condition="'${event-properties:item=Category}' == 'MyCategory'" action="Log" />
</filters>
</logger>
</rules>
However, it feels very clunky, especially having to restrict the categories with a maxLevel to prevent duplication, so I'm sure there must be a better way?