Problem is, the link you provided contains information about DataObjectManager instead of ModelAdmin (which this thread is about).
I keep ending up with "Unable to traverse to related object field"
So any other thoughts?

Hi

This works fine for me.

      
       static $has_one = array(
						'Advertiser' => 'Advertiser',
						'AdvertImage' => 'FittedImage',
	);	
	
	static $searchable_fields = array(
						'TargetURL' => 'ExactMatchFilter',
						'Title' => 'ExactMatchFilter',
	);
	static $summary_fields = array(
						'TargetURL',
						'AdvertImageThumbnail',
						'Title',
						'ToolTipTitle',
	);  
	
	public function AdvertImageThumbnail() {
	  return $this->AdvertImage()->CMSThumbnail();
	}

ta

Found the solution to my problem on IRC:
http://silverstripe.org/data-model-questions/show/263751#post263751