The idea of a keyboard on the visor sounds really slick. Until you start to think about it.
I have a friend that is going into the EMT business, and some of his EMT books show some of the things that happen during accidents.
Well, one couple was driving, hit another car head on at 45 MPH, ... he had a set of CD's on the visor. The weight of the CD's on the visor lowered it just enough for the visor to catch his head at the time of impact, this scalped the guy. It doesn't seem like much force, and the visor seems flimsy, but it's really dangerous. This is why you are always told to fold the visor back when you use it, so it's right next to the windshield, so that it can't scalp you.
Needless to say, this killed it guy, and the girlfriend lived.
SO, don't place anything on a visor! OR make CERTAIN it's got a VERY tight screw holding it and it takes ALOT of force to pull down.
I used to have a CD holder on my visor, and I have since removed it.
I also used to have a box of tissues on my rear dash, that's been removed too, because that can cause a serious problem, also.
My suggestion is to buy a RF keyboard, one with a power switch, and simply place the keyboard between the seats when not in use. This seems to work extremly well for me.
*Knight steps off his high horse.*