So Abraham got to work and adapated the example Page Not Found MODx Plugin to work as follows: domain.com/19 forwards to (301) domain.com/category/resource-19-alias.html .
The Plugin Code
Create a new plugin, name it whatever you'd like resourceRedirect. Then on System Events check OnPageNotFound.
<?php
if ($modx->event->name == 'OnPageNotFound') {
$output = '';
$param_alias = $modx->getOption('request_param_alias');
$get = $modx->getOption('GET', $modx->request->parameters, '');
$rId = $modx->getOption($param_alias, $get, '');
if(!is_numeric($rId)) {
41return $output;
}
$rId = intval($rId);
$resource = $modx->getObject('modResource', array('id'=>$rId));
if($resource) {
$url = $modx->makeUrl($resource->get('id'));
$modx->sendRedirect($url);
return $output;
}
}