Mansour Ijaz, a U.S.-Pakistani citizen, has offered to record his testimony abroad and submit it to the Supreme Court.
He says he fears he could be detained if he returns to Pakistan.
Ijaz has implicated President Asif Ali Zardari in a memo allegedly asking the United States to help thwart a military coup.
Ijaz says he was asked to deliver the memo to the U.S. administration.
Pakistan's government has denied any involvement.
Ijaz has also refused to appear before a parliamentary committee that is separately probing the memo, which has exacerbated the already tense relations between Pakistan's civilian government and the powerful military.
compiled from agency reports