I'm trying to add a custom block for my module into the head block. I would like this block to be added on every page. Now, my problem is that the block is not rendered at all except if I add the following into one of the files under the design/frontend/<theme>/default/layout/ folder:

<block type="page/html_head" name="head" as="head">
  <block type="core/template" name="my_custom_block" template="my_custom_block/custom_block.phtml"></block>
</block>

The layout file for my custom module is located under design/frontend/base/default/layout/my_custom_block.xml and looks like this:

<?xml version="1.0"?>
  <layout version="0.1.0">
    <default>
      <reference name="head">
        <block type="core/template" name="my_custom_block" template="my_custom_block/custom_block.phtml"></block>
     </reference>
    </default>
  </layout>

I can verify that this file gets loaded by adding a <remove name="root" /> to the layout definition, which in turn will trigger an error when trying to load the page.

Of course I could simply add the block directly into one of the themes templates, but it would be much cleaner if the block is visible just by installing and enabling the module.

Note that I have added the following dependency in app/etc/modules/My_Custom_Block.xml file:

<?xml version="1.0"?>
<config>
  <modules>
    <My_Custom_Block>
        <active>true</active>
        <codePool>community</codePool>
        <depends>
            <Mage_Page/>
        </depends>
    </My_Custom_Block>
  </modules>
</config>