I wouldn't advise using a keyboard in the sun visor, especially one with a trackpoint. Would seem pretty awkward to be typing and holding your arm up to move the track point. There's several small sized keyboards that you could
probably stow somewhere under a seat or have wireless so you or a passenger could use. The IBM Space Saver keyboard is nice and small. I bet even smaller if you hacked off the numberpad.
IBM Space Saver II
The keyboard that I use is perfect. It's got a track point in the most intuitive place. Very hard to find nowadays but when it came out it was dirt cheap.
very small too. Dimension 340 x 170 x 28 mm
yet another small keyboard, though just a bit bigger than your visor
oh another one here.
340mm x 163mm x 31.5mm.