There has been. Prime Minister Harper appointed Jason Kenney, who was a sitting member of Parliament at the time, with the role to assist in and secure the release and return of Michael Kapoustin, who had been detained in Bulgaria for at least 14 years at the time. Ultimately, Michael Kapoustin's lawyer Dean Peroff, and Jason Kenney, effectively acting as a special envoy, obtained the release and return of Michael Kapoustin. There's one example, but there have been others.
In the case of Maher Arar, I believe that Senator De Bané was appointed as some level of special envoy, to some extent. I don't remember all the details around the file and Senator De Bané's appointment.
It certainly has occurred, and the appointment would be at the ambassador level. Could it be inclusive of the two Michaels and other Canadians? It would make sense that it might be. I believe that would warrant some thought.
I think particularly with Huseyin's case, since he has been detained for so long and has languished for so long, and with the overlay of the persecution of the Uighur community in northwestern China, we now have a double dose in requirement.