Er... has the "50th Anniversary Special" even been written yet?
Last I heard, Moffat mentioned "having a few ideas", but nothing much more concrete than that.
Matt Smith has said he expects to see the script in February.
I suspect that, if the script involves past Doctors, that Moffat has a draft because he'd need something to show the actors to sell them on it. A multi-Doctor story is going to involve a lot more negotiation with actors than most any other episode -- past Doctor actors aren't simply regular guest stars, they have expectations and demands. That's why there's a version of "The Five Doctors" script that features Tom Baker heavily, because it was an inducement to get Baker to sign. That's why Peter Davison and Colin Baker weren't thrilled with Adrian Rigelsford's script for "The Dark Dimension" because it didn't give them anything to do. There are a lot more moving parts, critical
parts for a multi-Doctor story that need to get lined up. So Moffat's usual "write it at/after the deadline" approach isn't going to work. He'd have to write this one ahead of time.
Assuming, of course, that that's what Steven Moffat has planned for the anniversary.
Screw that... Captain Jack and River Song need a whole SERIES together.
Alex Kingston has said she wants to do exactly that.