I recently had more time to investigate my earlier problems with the GuruPlug. The problem was discovered by some people on the plugcomputer.org forum. The problem stock U-Boot is that is incorrectly reports the arcNumber. The solution then, is to upgrade U-boot!
After a failed flashing attempt(don’t attempt to tftp flash u-boot.kwb from within U-Boot!) I needed to use JTAG combined with the included JTAG adapter. To do this, make sure that the UART cable is unplugged, and the JTAG cable is plugged in(unplug/replug the USB adapter just to be safe). First grab the guruplug-installer package, then grab a known-good copy of U-Boot.