You really need to get into the habit of saying what type of Z you mean. The 5xxx types are obviously very different from the Cxxx types and your question is not really answerable without knowing which you are asking about.
For the clamshell C types, just tilt the screen forward about 10 degrees or so from verticle and the display backlight does an excellent job of lighting up the keyboard, even in pitch darkness.
I have never owned a 5xxx Z so someone else will have to answer you there.
If you are asking about a *software* hack ----- can't be done. Its a hardware thing. So getta pen light (G).