Thanks a lot ChemBoy, that put me on the right track. I still had to modify execute() to put the correct result set into the statement tracker, but now it seems to work fine.

Here's the change I made, for posterity (it's a godawful hack, but maybe it'll still be useful to someone at some point):

@@ -559,6 +559,10 @@
         my $dbh = $sth->{Database};
         eval {
             $session->verify_bound_params($dbh, $tracker->bound_param
+s());
+            my $idx=$session->{state_index}-1;
+            my @results=@{$session->{states}[$idx]{results}};
+            shift @results;
+            $tracker->{return_data}=\@results;
         };
         if ($@) {
             my $session_error = $@;
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