must admit Genghis will know the engineering implications better than I.
1. I also would be very wary of attaching anything which isn't in the original certification to the main control system. And would argue that a friction clamp to the yoke was actually an illegal mod. Your sticking a 2-3 kg weight on the main flight controls could set up all sorts of funny harmonics which wouldn't have been tested for. Also because of the height and weight of them it will effect the lateral balance of the controls ie effect the stability in roll.
2. The yoke clip board is a sharp edge right in front of your face and they are quite sturdy things when compared to your face.
The GPS yoke holders also fall under this bracket although they arn't as heavy as the clip boards. Would definatly advise you don't have them fitted in any other stage of flight other than cruise.