That would be cooler if we didn't know Capaldi was going to be 12 though.
I dunno. I know many people who watched "Asylum of the Daleks," knew Jenna-Louise Coleman was going to be the next companion, and didn't make the connection between Oswin and JLC. Of course, the difference is that 1) Peter Capaldi is better known than JLC was, 2) JLC wasn't introduced to fandom in a highly watched American Idol
results show like Capaldi was, and 3) the BBC would promote the Christmas special as the end of one Doctor and the start of the next, all of which would negate the surprise potential.
I am partial to the idea floated above that the regeneration should happen mid-episode (I'm guessing somewhere between the 40 and 60 percent mark) so that the episode can end on a note of triumph rather than tragedy. However, Downton Abbey
ended its Christmas special last year with Matthew Crawley bleeding out, so maybe the British public tolerates downer endings on Christmas Day.