I would say that it's very difficult to believe they could keep McGann appearing in Season 7b a secret, and even harder that they might keep an upcoming appearance from Tennant a secret during filming, but then again, they managed to keep Coleman a secret.
Coleman in "Asylum" would have been an easier secret to keep. First, all of her shots were studio-bound. Second, she may not even have filmed her scenes at the same time as the rest of the episode.
EDIT: 50th Anniversary Official News tomorrow for definite.
Odd day for an announcement. They have "The Bells of St. John" going out, so why waste two things -- the episode and the news -- on a single news cycle? From a messaging standpoint, Monday would be the better day to break the news. They can trumpet the success of "Bells" and then segue into their 50th-anniversary announcement. That would be two news cycles.
No, this timing is suspicious. Either it's a delayed reaction to the Private Eye
report or they're trying to get in front of a different story.