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Maybe. Not sure the room, height-wise, that’s available in the FWL13. For the FWL16, the available height is known.
You might sacrifice some stiffness. I would guess that they created the input cover the way they did, holes through the cover for individual keys, to lend some stiffness, structural integrity to the base.
For the usb connection to a FWL16 keyboard, you could add a usb hub chip to share the port for the fingerprint reader, which passes through the touchpad board.
I’ve wondered if Framework might do that, a few generations down the line, perhaps. Presuming, they are eventually satisfied enough with the reliability of the FWL16 keyboard setup. That might be something that works better in some future model in between the 13 & 16. Keeping the 13 as thin & light as possible.