Disable hardware keys on Cyanogenmod 12.1 / SM-T800 (Samsung Galaxy Tab S)
I got an answer for this on this xda developers thread:

Isnt the key file in cm called something different sec_touchscreen.kl or something or other? 


Why yes ashyx, it is! In fact, it only has two entries! One is for the app switcher button and the other for the back button. After commenting out these lines, the buttons were disabled.

After digging through the settings, I also found that the button back lights are adjustable in this build of Cyanogen and was able to turn them off.