Good points both Karnak and Midway - No point in implementing something that won't work right from the word go, so I zoomed the clipboard all the way out, which makes it of about the size it would be in the knee position. I then progressively reduced my display resolution all the way down to 1024x768 and evaluated the readability. Long story short, the kneeboard text is legible at all resolutions, even to my aging eyes. The map display is also fine - though I'll caveat this to say that I usually fly with it zoomed in to show just the closest 4 sectors. If you have the whole map, it gets a bit cluttered. I also have to say, it feels quite natural and intuitive to be looking down for the map exactly where I'd normally have my map.
A couple of points did come up in this admittedly simple test, both mouse-related. One is that the current kneeboard always displays "flat" on the screen, and this is a good thing for mouse clickability. It might awkward for clicking at the 130-or-so angle that it would be in the knee position (or maybe not, but we're all used to the flat surface paradigm for mouse movement). The second is that at this size it takes more precise clicking, which some might find finicky. Both these and any residual readability concerns can be addressed by simply letting players choose plane-relative or view-relative display for the clipboard, with the ability to set the plane-relative co-ordinates as they like. This also takes away a lot of the work in having to set the knee-position for each plane - so it's a win-win.
@Karnak - you also have a good point concerning how much this would be used. However as the game becomes richer, it's inevitable that more and more features will be used by fewer and fewer players. How many pilots use the GUI options to alter the preferred radar colors for friendly and enemy? Yet there the options are, taking up clipboard real estate. How often does anyone actually see anyone's custom nose-art? Yet the custom nose-art code exists, and it's cool to have it. I can't say how many people would use plane-relative kneeboard option (though I think a lot of more advanced pilots will find it useful) However I can say for sure it's a more meaningful and useful option than allowing enemy radar blips show as purple rather than red.
And again, it's available for a very small coding effort, which makes the cost/reward ratio very attractive. Just not having to listen to me go on about it would be worth the effort all on it's own!
Cheers!
Paul