Fenom

base.tpl

{set $properties['tpl'] = $_modx->getPlaceholder('template.tpl')}
{if empty($properties['tpl'])}
    {set $properties['tpl'] = "default"}
{/if}
{include "file:templates/tpl/{$properties['tpl']}.tpl"}

plugin

<?php
if($modx->context->key == "mgr"){
    return;
}
switch($modx->event->name){
    case "OnLoadWebDocument":
        if(is_object($modx->resource) && $modx->resource->id > 0 && $modx->resource->template > 0 && $template = $modx->resource->getOne('Template')->getProperties()){
            if(is_array($template) && count($template)){
                $modx->setPlaceholders($template,'template.');
            }
        }
        break;
}