upon a cursory glance it seems that the singlepage has to exist at:

define('DIR_FILES_CONTENT_REQUIRED', DIR_BASE_CORE . '/single_pages');

which would be:

define('DIR_BASE_CORE', dirname(__FILE__) . '/..');

which is one directory up from concrete/config/base.php where this stuff is stored, which means it is the concrete folder, so it has to be present in concrete/single_pages

I got this from going to the page model and looking at the isSystemPage() method.

There are probably other ways to do this but I am in the middle of something else :(.

Aha! That makes sense. So "system pages" really refer to internal/core Concrete single_pages?

I'm wondering what the best practice would be for creating a completely hidden "page" within Concrete.

I don't know if this is the right question... is there a way to register controllers that have no views/templates?

For example, I've created a controller that just handles a few forms and AJAX requests. But my single_page and template are empty (on purpose). My controller actions all end with redirects or JSON output directly to the browser.

i dunno i am about 5 days away from this, I'd like to settle on a tools file that extends controller and just post to and get from that, but I am not sure what exactly will happen there. I know you can set the controller for a view in code but again not too sure until i get into it.

Simple json $.get is easily handled through a tools folder and most of the concrete5 if not all of the api is available there since it goes through the dispatcher anyways.