More investigation gives that if I do declare the exception in pages.xml as this:
<exception class="java.io.NotSerializableException" log="true" logLevel="error"> <redirect view-id="/feedbackPage.xhtml"/> </exception>
... I do get a full an detailed stacktrace for this in the logs. As expected.
What I didn't expect was that action seems to be required to get essential logging going. Shouldn't it be the other way around - i.e. suppressing those Exceptions which you know is ok?