Could a chronic cough be one of the biggest symptoms in someone with lupus? even with a clear chest x-ray?

Not generally: Pulmonary involvement in sle is generally manifested as pleural effusions(fluid around the lung) and pneumonitis. These would generally result in an abnormal chest x-ray. Chronic cough in sle can be associated with esophageal reflux which is not uncommon and generally associated with raynauds.

Cough: Cough not a common sx of lupus. Look to other causes.