That said, I can envision a scenario where if the boy still can't speak AND Casca doesn't recognize him AND Guts is hopelessly clueless, they could use the Corridor of Dreams to ascertain his identity and learn more about him. Then they might find another mind there, someone else's dreams... dun dun duuuun

. That would be insanely cool.
I had originally planned to include this same scenario in the previous post, but I ended up leaving it out, because I thought it might be a bit too much. I'm really glad it occured to you as well, I guess that means it's not completely bonkers. Going by the same set of rules used for Casca's ceremony, her and Guts could, in theory, enter his dreams, because he feels close to them, in fact they are probably the only ones who could. This would of course be extremely interesting to see, it would be greatly revelatory for both of them, and if you added Femto to the equation, even more so.
The main problems with this idea that I can see: First of all, the fact that there aren't really any obviously good reasons for them to subject him to the ceremony, other than for curiosity's sake, which I feel is unlikely. Then, there is the uncertainty about how Femto's ego actually coexists with the boy's. They are both sharing the same brain, so from a mental standpoint, this could be something akin to a multiple personality disorder. It would be pretty funny to imagine that somewhere in the deepest parts of the brain there is a dark chamber with 2 chairs where Femto and the Moonlight Boy sit, each awaiting their turn like you often see depicted in other popular media. Maybe it's possible to traverse between the two, maybe not. There is no guarantee they would find out the whole truth, even if they were to explore his dreams.
That's why I wouldn't bet too much on the Corridor of Dreams ceremony being the means by which they find out about his dark secret, but I could see a scenario where they use a comparable type of magic (according to Schierke, the same principle is used for ceremonies that enable magicians to reach deep into the Astral World) to somehow try and combat Femto's spirit, maybe subjugate it temporarily, allowing the boy to take full control for a longer period of time, possibly allowing him to think more freely and even speak (perhaps his lack of speech has to do with Femto limiting him in some way, for obvious reasons). This thought process can go on.