Is it possible to meditate on the impersonal? Not really. Srila Prabhupada explains why:
Impersonal meditation is a bogus invention of modern days. In none of the Vedic literatures is impersonal meditation recommended. In Bhagavad-gita, when meditation is recommended, the word mat-parah, which means "pertaining to Me," is used. Any Visnu form pertains to Lord Krsna because Lord Krsna is the original Visnu form. Sometimes someone tries to meditate upon the impersonal Brahman, which is described in Bhagavad-gita as avyakta, meaning "unmanifested" or "impersonal." But it is remarked by the Lord Himself that those who are attached to this impersonal feature of the Lord suffer a very troublesome task because no one can concentrate on the impersonal feature.