Just for the heck of it, I took a look at a bunch of the old Ytsejams from the time that the album title and first single were announced. Here's what I saw:
• According to one Jammer, a week before MP announced the album title, he read on Billboard's new release roundup that the title was Metropolis part 2: Scenes From A Memory. However, no one seem to pick on this post or comment further about the possibility that the album as a whole might be a sequel.
• There was lots of talk about the "veil of secrecy" that had been imposed on the list.
• Some schmuck that tried to sell tape copies of the album. A couple others commenting about having heard the album or at least several tracks, but didn't say anything about it being tied to Metropolis pt I.
• One Jammer who ran a Fates Warning message board created a special place just for uncensored discussion of album #5 before it's release. Dunno what all was discussed there, but surely whoever went there read about it being a sequel.
So it seems that anyone who hadn't heard the album at that point (probably most of those on the list) still didn't have a clue that SFaM was Metropolis pt. II.
Nice! what a time it must have been, I was already a fan but I still didn't follow actively the various bands, also because I was still without a proper internet access so all I had to go by were bits and pieces here and there (I have a flashback of being with the class at a technological fair, that was happening annually in Milan, and trying to print off the web lyrics for Manowar and Bruce Dickinson songs...), so I missed all the speculation. I had a record store right on the way back home (I could come down the subway, go up, enter immediately in the store which was quite nearby and then come back) so that's where I probably must have seen the release date, if I remember there was just a board with the name of the bands and the release date.
In this day and age you can still surprise people, and the buildup for The Astonishing was quite cool, but keeping basically everyone in the dark about the title of an album and its actual contents was impressive. Of course someone must have known, and the more the release date was approaching, more people were starting to know, but overall the veil of secrecy basically worked just fine. With a little help from the internet being in its relatively young days, but still it worked