Obviously, the official flashers will not allow you to crossflash. You need an unscrambled firmware (*.bin file) which can be flashed using the unlocked flasher method: http://club.myce.com/f87/how-upgrade-your-asus-bw-12b1st-liteon-ihbs112-2-a-331630/index3.html#post2678608
Make sure that you backup your EEPROM first and keep it safe.
Also, since there aren't any BD-W512GSA firmwares available, you might want to backup your firmware first, because without a valid firmware dump there won't be any way for you to flash back to a BD-W512GSA later on if needed. Since this drive is highly likely to be a 2nd generation drive, the Flash Utility or LtnFlash 2.6.11 won't be able to correctly dump your firmware (the dump created will contain just random data), so you might want to use dosflash to create a firmware backup of your BD-W512GSA firmware. See here: http://club.myce.com/f44/liteon-ihas124c-crossflashing-firmware-333397/index2.html#post2696164