Eccleston won't be appearing. BBC issued a press release saying he wasn't appearing, didn't they? You don't do that and then have the guy show up.
He ain't showing up.
Prior to the release of 'Night of the Doctor', there was a statemen from the BBC which stated that no past Doctors other than Tennant were going to appear in anything related to the special, including mini-episodes (a curiously specific denial in hindsight, it must be said), with some of these sources even emphatically underlining each of the individual actors in turn, including Paul McGann, as being somebody who was not going to appear.
Basically, the makers of Doctor Who lie
. And they're happy to be liars, because there in lies the art of truly surprising the audience.
Having said that, if I were placing odds on Eccleston's secret involvement... I'd say it is pretty unlikely. The man himself has stated he has no interest in it and he's a straight-shooter when it comes to these things. I doubt he'd be willing to play the publicity game either, keeping his own involvement a secret, especially after the disrespect the BBC PR department shown for him publically when he left the role.