I don't know, if anet notices someone talking about how they got an invite five minutes after they sent it out, and that the person signed it after five minutes and thirty seconds, would anet care that they hadn't signed the NDA yet? Frankly, I doubt it.
How they handle the NDA is up to them, for all we know they won't enact it even if people talk about bugs they encountered, but then why have the NDA in the first place? No need to try and find every loophole, especially when the NDA isn't viewable to anyone who didn't get in the closed beta in the first place.
Best to be safe and not tell people in public, or message guild wars 2 on facebook "thanks for the beta invite".