Got it!
In robot_functions.php, I've added a test at the end of function phpdigDetectDir.
This is how I've done it for the test I need, showing lines 537 onwards. My addition is at line 543:
//test the exclude with robots.txt
if (phpdigReadRobots($exclude,$link['path'].$link['file']) == 1
|| isset($exclude['@ALL@'])
) {
$link['ok'] = 0;
}
//exclude if specific variable set
if (strpos($link['file'],'print=y')) {
$link['ok'] = 0;
}
//print "<pre>"; print_r($link); print "</pre>\n";
return $link;