The radio might not be exposed to the OS. If it's a Fullmac device, you usually don't have access to the radio stuff. Even with Softmac, there's not a guarantee.

I am sure that's a problem on the client, but on the AP, the manufacturer controls both the hardware and the OS.

Not necessarily. Again with fullmac devices you don't control the complete hardware.

Yeah but if you are Cisco, Netgear or DLink, and tell your supplier that you want a function to access noise in the signal, or you want to get a true random number based on that noise, I am sure they could accommodate in future generations of their chip at an insignificant cost.