I did some poking around and the NX-Refit Enterprise is outside. Freeze frame can catch it. Or anyway some Refit of that class. *I* love the Excelsior, but it or Enterprise-B wouldn't have the punch you are looking for. E would. And F would. I really don't think it is in keeping with what they are doing there to rebuild Enterprise-D. They are keeping relics here, but probably don't have the budget to build an entire Starship just to look at. WHY what is in Bay 12 is in Bay 12 is PROBABLY not to keep it a secret in story. This is a museum. So just from your list, I would deduce Enterprise-F. We were already spoiled that F is damaged and out of service, so likely it is in Bay 12 because it is being repaired and isn't ready for display. I will watch the scene again to get solid context. From where I am right now, I don't agree with your idea that what is in Bay 12 would be the lynchpin because it isn't networked to the fleet. I really don't think ANY of those ships would be on the network. They aren't intended to be used. There are 2 Klingon ships and a Romulan Bird of Prey, too. Why would those be on the network?
If Bay 12 IS a secret it is a private project of Geordi's and then it could be anything. But probably a giant scale replica of Leah Brahms.
Bay 12 has something in it that will help them travel without being tracked, or at least Geordi's daughter thinks so. I am assuming its a hero ship, and I like the idea of a salvaged Enterprise D a lot more than the F or even the E. Sure, the F and the E are a lot more powerful ships, but the Enterprise D is probably the second most Iconic federation starship behind the original TOS enterprise. The F hasn't ever been seen in canon other than the trailer, so it might be getting repaired elsewhere since that ship is a new and an active class starship compared to the probably all retired by this point Galaxy class. The Enterprise E, as much as I loved it twenty-five years ago, didn't get the love it deserves, with it only being in three movies and some video games, so it showing up probably isn't happening. With you catching the NX refit class outside, it probably won't be that ship either. I doubt is either of the Excelsior class ships for the same reasons as I don't think it will be the Enterprise E. Which makes me think, Enterprise D is being repaired and rebuilt.
Having Geordi in charge of the Enterprise D's restoration/reconstruction would make sense since he rewrote the book on the Galaxy class, and doing so in space dock would be completely feasible and not remotely out of the ordinary since they used to do repair and construction work inside of Space Dock before they replaced it. If Starfleet can turn a Luna class ship into a neo constitution class, they can get the aforementioned saucer section and pair it up with a stardrive section. Considering how many Galaxy class ships were damaged in the Dominion War and that whatever Galaxy class ships that were left would have been all retired or regulated to light transport service by now, there should be plenty of parts to match up and repair the D.
However, all this speculation is getting my hopes up, so let's just see where they go with it.
Having Geordi in charge of the Enterprise D's restoration/reconstruction would make sense since he rewrote the book on the Galaxy class, and doing so in space dock would be completely feasible and not remotely out of the ordinary since they used to do repair and construction work inside of Space Dock before they replaced it. If Starfleet can turn a Luna class ship into a neo constitution class, they can get the aforementioned saucer section and pair it up with a stardrive section. Considering how many Galaxy class ships were damaged in the Dominion War and that whatever Galaxy class ships that were left would have been all retired or regulated to light transport service by now, there should be plenty of parts to match up and repair the D.
However, all this speculation is getting my hopes up, so let's just see where they go with it.