This question has been answered by BobRay. See the first response.
[[!AllowPrivate]]
<?php
/* AllowPrivate snippet */
if ($modx->user->isMember('AllowPrivate')) {
return '';
} else {
$loginPageId = 12; /* replace with ID of your login page, or some other page */
$url = $modx->makeUrl($loginPageId, "", "", "full");
$modx->sendRedirect($url);
}<?php
$user = $modx->getUser();
$username = $user->get('username');
$c = $modx->newQuery('modResource');
$c->innerJoin('modTemplateVarResource','TemplateVarResources');
$c->innerJoin('modTemplateVar','TemplateVar','`TemplateVar`.`id` = `TemplateVarResources`.`tmplvarid` AND `TemplateVar`.`name` = "allowUsername"');
$c->where(array(
'parent:IN' => array(21),
'deleted' => false,
'published' => true,
'TemplateVarResources.value:=' => $username,
));
$resources = $modx->getCollection('modResource',$c);
$output = '';
foreach ($resources as $resource) {
$output .= 'Here is your private content:<br/>';
$output .= $resource->get('content');
}