Finally I find a workaround. it works on my T420 with esxi 5.0u2
your hardware has to support vt-d. both motherboard and cpu.
turn on vt-d in bios, and then make onboard vga as pci passthrough device in esxi configuration. after a reboot, the esxi will freeze as dead at some point, don't panic, it still running.
remote login via vphere client, assign that pci passthrough device to any virtual machine(one is enough). when that virtual machine is powered on. the screen will turn blank.
although the vga passthrough will never work, 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.
drawbacks: you can't use console anymore. if you want to restore. just delete that pci passthough entry, make it NOT passthrough and have a reboot.