Thinking about it, the BBC has long been fond of prequels - they did ones to Last of the Summer Wine (called First of the Summer Wine) while that show was still airing and one to Only Fools and Horses called Sex, Drugs, Rock & Chips after OFAH ended its run. Across channels, ITV is about to show an Inspector Morse prequel called Endeavour.
McGann is not a box-office draw but his presence in a tv drama does tend to attract notice and a certain cachet. So something like this is not entirely inconceivable but I still think it's very unlikely. Probably the best opportunity would have been,as Christopher
suggested, when Tennant was doing stagework, as a sort of stopgap.
If McGann does appear in the 50th special and there's a positive response to his Doctor and a similar situation arises in the future (Matt or his successor taking a year off for movie or other commitments), I could see this happening. But that's still a lot of 'ifs.'