How to programatically design mode select a component to be included in the sidekick

The Problem

You want to have a component programatically available to authors without having to enter design mode and add a component for the template.

The Solution

Go to your relevant template's design mode. For this example, we will use the Geometrixx site, located at:

/etc/designs/geometrixx/jcr:content/contentpage/par

Then edit the components property to contain the path to your component, for example:
/apps/geometrixx/components/customcomponent

Save, and reload the editor, and you should have the component added to your list.