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; } }