Just thought I'd report success. I couldn't get the TFTP method to work (other users have reported problems as well), but since the process for generating the image is the same for the v5 as the v4, I generated a firmware update image and uploaded it through the web ui.
The message detailing methods to reflash the firmware is here. The critical section was
> dd if=tpl.bin of=boot.bin bs=131584 count=1 > cat owrt.bin >> boot.bin
boot.bin uploaded through the usual flash a firmware method.