This is an image from the framework website, but copying it here because it’s a very clear picture of the space we’re working with on the FW16. Any “too tall” keyboard (like the X1 Carbon I’m working with) needs to navigate the row of pogo pins running across the center of the usable space, as well as the white brackets on the sides. On the other hand, just above that is some extra depth that could be exploited for your driver chips, FPC, etc.
In another thread, @Josh_Cook mentioned a plan to create a facsimile of the complete upper deck of the FW16. That would be super-useful for Input Module developers, even if it was just a STEP or STL file we could 3d-print as a mockup or import into CAD.