Once you've grabbed them edit mn700.txt and replace two @@MAC@@ occurences with your device MAC address (printed on the board sticker, should start with 000D3A). The MAC string entered to the mn700.txt should be as following: 00:0D:3A:xx:xx:xx
Once done run nvserial like this to produce your image:
nvserial -i mn700.bin -o cfe.bin cfe.txt
The resultant cfe.bin file is the thing you are looking for. :-) Use JTAG to flash it to the device. When flashed correctly, the Power LED should start blinking on power on, indicating recovery mode. If it does not blink - try reflashing image and clear nvram.
Once done use ASUS Firmware Restoration Tool for the WL-500g router to flash custom firmware.