I believe that to do this without using any of the existing firmware you would just want to grab a JTAG programmer and flash your own version of their firmware, or a custom blend of something. Flashing it is easy and they've got plenty of instructions on the web.
You can actually do this with a usb cable, or flash your own firmware over the air, you'd only need a jtag programmer if you wanted to replace the open-source bootloader :)