gistfile1.txt

// config.xml
    [...]
    <frontend>
        <layout>
            <updates>
                <foobar>
                    <file>foobar.xml</file>
                </foobar>
            </updates>
        </layout>
    </frontend>
</config>

// foobar.xml
<?xml version="1.0"?>
<layout>
    <adminhtml_index_login>
        <reference name="form.additional.info">
            <block type="core/text_list" name="foobar" template="foobar.phtml" />
        </reference>
    </adminhtml_index_login>
</layout>

// foobar.phtml
Hello World