Currently there is one item which can be customizable and this is
the list item.  If you need to ask the user extra things, such as
to pick from a list of places to log into, or set of custom login
sessions you can setup the list item and add listitem children that
describe the choices.  Each listitem must have an id and a text
child.  The choice will be recorded in the file
<ServAuthDir>/<display>.GreeterInfo
as <list id>=<listitem id>.
For example suppose we are on display :0,
ServAuthDir is <var>/gdm
and we have the following in the theme:
<item type="list" id="custom-config">
<pos anchor="nw" x="1" y="1" height="200" width="100"/>
<listitem id="foo">
<text>Foo</text>
</listitem>
<listitem id="bar">
<text>Bar</text>
</listitem>
</item>
Then if the user chooses 'Foo' then
<var>/gdm/:0.GreeterInfo will contain:
custom-config=foo
