The Template selector is done in JavaScript. It gets the list from an Ajax call to the template getList processor, so you'd have to rewrite or override the MODX template getList processor: core\model\modx\processors\element\template\getlist.class.php
You could possibly set the values in the Template selector drop-down with JavaScript that runs after the MODX processor is finished, by unbinding the drop-down's onclick function and adding your own, but it would be very difficult.
FYI, in your code, $modx->resource is usually not set in the Manager, so it would be just $resource. Resources have a 'template' field but not the field you're using, so set() would fail, and if it didn't, you'd be seeing the resource's template to an array, which would also fail.