Recently, I searched how specify the start ID of the breadcrumb and it seems that this is not possible.
So here my (dirty) solution.

Open the
/core/components/breadcrumbs/breadcrumbs.class.php file and add these few lines :
In the
__construct function, add the config parameter
startId :
<?php
$this->config = array_merge(array(
'startId' => null,
...
),$config);
In the
run function, replace
<?php
$resource = & $this->modx->resource;
by
<?php
if(!is_null($this->config['startId']))
{
$resource = & $this->modx->getObject('modResource', array('id' => $this->config['startId']));
}
if (empty($resource) || $resource == null)
{
$resource = & $this->modx->resource;
}
Maybe Shaun could add this feature in a future release of Breadcrumb !