There's no precluding anybody, or the former government, from offering an apology. As a matter of fact, Justice O'Connor raised the question of the inability of the government of the day, when Mr. Arar was in prison, to act with one voice--to say he was a person we were not worried about in terms of security and criminal background.
We had a case in which the Prime Minister apparently at one point wanted to send such a letter. The Solicitor General didn't agree to sign it, and if the security people weren't signing it, that had some impact, according to Justice O'Connor, on the Syrian officials. Justice O'Connor says there were statements made by different members--ministers, at the time, in the former government--that resulted in some ongoing grievance.
I think in those cases, individual members who are no longer with the government would have to decide on their own if they think an apology is forthcoming from them individually.