In case you are interested ... ;-P

The replies are generated in list replies

    
my $orderby= $VARS->{notesortorder}
       ||  $settings->{orderby}
       ||  "createtime";
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The sort order is labeled in shownote

  24: my $orderby= $VARS->{notesortorder}
  25:    ||  $settings->{orderby}
  26:    ||  "createtime";
  27: 
  28: my %orderby_labels = (
  29:     'createtime'      => 'Oldest First',
  30:     'createtime desc' => 'Newest First',
  31:     'reputation desc' => 'Best First'
  32: );
  33: 
  34: my $replies_info;
  35: if ( $orderby_labels{$orderby} ) {
  36:     my $fmt_replies_info = htmlcode( 'formatRepliesInfo','', $NO
+DE );
  37:     if ( $fmt_replies_info =~ /^[^0]/ ) { # something other than
+ zero
  38:         $replies_info = qq(<span class="note-ordering">Replies a
+re listed '$orderby_labels{$orderby}'.</span>);
  39:     }
  40: }
  41:
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List replies is called later.

47:       htmlcode('list replies')
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So I think you can change line 38 to show a select box changing $VARS->{notesortorder} and performing a reload.