Well knock On is a kernel-reliant feature. You will need a ROM or kernel that supports it (none that I know of for the Kindle Fire exist). You could keep the Kindle connected to USB power and enable the option in Settings->Developer Settings->Stay Awake to consistently keep the screen on. Otherwise I would suggest something to make the power button easier to press (perhaps a lever of some kind). Disabling the lockscreen is also a good idea for this kind of project (go to Settings->Security->Lockscreen->None). I'm drawing a blank otherwise because I always use my Kindle as a normal tablet.
Sent from my ceiling fan using XDA ‹bermensch -5
HTC Vivid (AT&T; running Resurrection Remix KK)
Kindle Fire First Generation (running OtterX bootloader & Nightly CM11 builds by Hashcode)
HTC EVO 4G (Sprint; running CM7 official build)
LG Optimus F3 (T-Mobile; running stock rooted 4.1.2)
iPhone 4S (blah; Sprint; running jailbroken iOS 7.0.2)
Nook Simple Touch (two of em; one is rooted and the other is not; B&N OS 1.2.1/Android 2.1)