Moffat said he wants to "have the box set, all the regenerations", and there is another Minisode called Last Day which will be on the DVD...
Dramatically and structurally they have to show Hurt's fate unless they're going to have him return in the future - and showing his fate means they either have to have Eccles back to show that regeneration, or fake it with stock footage as they faked Hurt in The Night Of The Doctor. So they have to do that either in the special or the minisode.
Unless of course Moffat just wants something to leave to wind up the fans. Which he might.
Well guess we could have a regeneration without Eccleston, have Hurt begin to regenerate then fade away, maybe pan away from where he's mid regeneration to Nine's leather jacket?
Even if Eccleston did come back we would still be technically missing a regeneration though, wouldn't we? I mean we never actually saw Two turn into Three did we...