Is it possible to limit the level at which entries can be selected from within an entries field? I have an entires field which is using a structure as a source. 
The structure has 3 levels and I would like to be able to limit it so the user can only select entries at the second level.

I've had a look through the docs and at the field settings but it doesn't seem to be doable. If anyone knows a work around or a plugin which can do this that would be great. I wonder if it is possible to create a plugin to extend this field to do this if not?
There's already a popular feature request for this:
"Entry Type" setting for Entries fields

But yes, it's currently also possible to do this by extending the default entries field type
and adding your custom criteria with getInputSelectionCriteria().

class BusinessLogic_CustomEntriesFieldType extends EntriesFieldType
{
    public function getName()
    {
        return Craft::t('Entries (custom)');
    }

    protected function getInputSelectionCriteria()
    {
        return array(
            'level' =&gt; 2
        );
    }
}