As usual, use it at your own risk. I am not to be held responsible for your actions.
Supported devices: WNR1000v2 (tested by me), WNR1000v2-VC (provided by Comcast for free; tested by Douglas Fraser)
Notice: Upgrade WNR1000v2 to the latest factory firmware: 22.214.171.124. Otherwise, the power LED may not behave properly.
You need to place the device into failsafe mode. For booting into failsafe mode, you need to power up the device while holding the reset button with a pin. The power LED should have an amber colour. Hold the button until it is starting to flash green. It starts to flash green after it flashes the amber LED for six times. After that, the device is in failsafe mode, accepting a firmware via its TFTP server. The device should respond to pings at 192.168.1.1, although the responses may be malformed.