There was some talk earlier about them opening the Hollywood Studios loop and AoA/Pop loop first. So if they wanted to soft open it, they could put a sign at the Hollywood Studios station, thereby enabling people heading back to their resort to use it. They could also put signs at the stations at those two resorts as well; some people are bound to walk by the sign going to or from breakfast. The fact that only a few people would know about the soft opening initially is actually a good thing: It means that it won't be inundated with guests and can open softly.
Yes. The minute it happens, it'll be all over the Internet, and although that won't get the news to every guest, it'll get the news to enough guests to be (again) a soft opening.