I've signed one or two passport *applications* as a doctor (of philosophy, not medicine!), but not sure about certified copies. As Joe says, I'd suggest checking with the company in question - I did recently sign something that was rejected as they had a very particular list. Stupid really, as he's never met his bank manager or GP, and has no accountant - and those were the only people allowed to say "yes, I know Mr Smith, this really is him"!
Hmm, reading down that list above, there's some odd ones on there. I could also qualify as a "professional photographer" - well, I have sold about a dozen photographs and got paid for them!