This probably rules out the classic doctors in any potential multi-doctor special. At least anything more than a blink-and-you'll-miss-it cameo. As I first thought if they decide to bring back previous Doctors it'll be Tennant and maybe Eccleston if he's willing.
I can see arguments both ways -- that it rules out "classic" Doctors for a potential multi-Doctor anniversary special and that it doesn't.
In the past, Big Finish couldn't do things that the television series was going to do. Hence, in one story with the Master they couldn't say that the character was the Master because the Master was going to appear in the series. Cardiff can say -- and has -- "No, licensee, you can't do this because it steps on our toes." Thus, the conclusion that "classic" Doctors won't be appearing on television in a multi-Doctor special in 2013 isn't an unreasonable one because Big Finish was given permission to do this.
On the other hand, it's possible that Cardiff knows that fans want to see and hear Doctors getting together and doing Doctor-ish things and decided that "stepping on toes" was okay because it's the anniversary.
To be honest, the former possibility is the one that I feel is the right one -- no pre-Eccleston Doctors in a televised anniversary special.