This is one scenario that may work, I would imagine everyone can come up with others. This isn't difficult to keep under wraps until delivery. The Signature Series holders make it very possible.
There's one major problem with this: reservation holders who cancel at reveal because of something minor like a color choice or interior option, but still want a Model X, will then have to re-reserve, which could put them at the back of the line (well into 2016). Tesla could get around this by putting them in the front of the line for general production, but that might make some people on the general production wait list upset.
It is (1) bad customer service to keep buyers in the dark about exactly what they are buying (2) useless from a competitive standpoint, because any design features would take years for other companies to emulate, given the long cycle of designing and producing a car and (3) needlessly complicated. Reveal event -> Design studio -> Ship.
I don't expect Tesla to seriously keep the entire car under wraps until deliveries start. Is there any good reason for this? I think it's a stupid idea and I would say this to Elon's face if he really wanted to go ahead with it (hypothetically, because based on the conf. call I don't believe he meant that the whole vehicle would be kept secret, just certain features).
Alternatively:
Tesla could release a configurator on its website, but have it password protected and accessible only to Sig series reserve holders. That way, the first customers could be assured of the vehicle config that they want, without details being made public.
However, this again begs the question of why secrecy is necessary. It seems pointless.