Finally I find a workaround. it works on my T420 with esxi 5.0u2
your hardware have to support vt-d. both mb and cpu.
turn on vt-d, and then make onboard vga as pci passthrough device. after reboot, the esxi will freeze as dead at some point, in fact, it still running.
remote login via vphere client, assign that pci passthrough device to any virtual machine. when virtual machine power on. the screen will blank black.
although the vga passthrough never works, but we get what we want: screen is blank. will never burn LCD anymore.
after that you will find a way to turn off the backlight. such as close the lid. or something others.