Great information, but a couple of points..
That firmware is source code, of course. I'm sure it is buildable into a .bin, but sharing the firmware file isn't an issue. Just pm and I'll send you a copy of the .bin
If you are lucky, your SB6120 might have a serial header on it. In that case you only need a serial console wire, and a computer running terminal software and a TFTP server. I'm happy to help you with the steps if you can get yourself this far.
Most SB6120 units do not have a serial header, and in that case you'll need something that can program the SPI flash directly. Hence the USBJTAG NT I use, or the flashing rig that mlord developed.