Unfortunately, this would make for a pretty expensive board. I talked to a pretty well-known game printer recently and they had a very hard time figuring out how they would even go about producing this, and felt certain that it would drive the cost of the game up substantially. It's a great concept, but is there an alternative?
I think there's actually quite a simple one. We currently have 12 pieces of information on each solution card, arranged in a 3x4 grid. We could instead simply arrange the information in a 12x1 grid (ie, a row) along one edge of the card. Then, instead of separate slots, we would have "The Interrogator", which is just a card sleeve with two windows, one on each face. Say you want to dig in London. Just slide the solution card into the Interrogator, face-up, until "London" shows through the window.(*) Then, flip the Interrogator sleeve over and look at the window in the back, which will either say "Yes" or "No" or whatever.
(*) This is different from the current scheme where each location essentially has its own "sleeve" -- here the possible locations are printed on the card. This does take up more space, but it also makes the interrogator enormously flexible, as it can handle any kind of solution element we want to include, including ones we haven't yet thought up.
The particularly nice thing about this is that I think that it removes the need for the red masking, since the secret information can stay on the side of the card that's face-down. This is good for two reasons:
(i) The "answer" doesn't have to be text -- it could be a picture, an icon, whatever.
(ii) The red masking wouldn't work for color blind players. Since the game is intended to be playable in solo mode, it's preferable to have an approach that doesn't rule out color blind players being able to play without the aid of a non-color blind player.
Additionally, we're not limited to 12 solution elements anymore; although 12 is about the right number, with adequate spacing, for a 3.5" card. But if we just have to have 18 elements for some reason, we could simply make the cards longer. (Since the solution cards aren't handled/shuffled too much, this wouldn't be that big of a pain).
This isn't the most pressing thing to implement near-term, but it is something we could think about if we could to a point where enough changes have accumulated that a prototype change is in order.