Can Deep Daan be performed for the souls of departed ancestors?

Yes. The Garuda Purana explicitly prescribes lamp donation as an offering for the welfare of departed souls. Deep Daan during Pitrupaksha — especially on Sarvapitru Amavasya — is considered most beneficial for ancestors. Lighting a lamp in the name of a departed person and floating it on a sacred river is a time-honoured way of offering light to the soul in pitru loka.