Expose a modules crontab schedule to system configuration (Magento 1.x)

config.xml

<?xml version="1.0"?>
<config>
  ...
  <crontab>
    <jobs>
      <[namespace]_cron>
        <schedule>
          <!--
          /**
          *
          * The magic.
          * Exposes the schedule to system config.
          *
          * Now you can add system config for your module
          * and change the schedule as you see fit.
          *
          */
          -->
          <config_path>path/to/config_var</config_path>
        </schedule>
        <run>
          <model>[namespace]/observer::method</model>
        </run>
      </[namespace]_cron>
    </jobs>
  </crontab>
  ...
  <default>
    <path>
      <to>
        <config_var>0 * * * *</config_var>
      </to>
    </path>
  </default>
  ...
</config>