[robespierre]

The boot environment is what runs from the EPROM on the mainboard (firmware). If, after pushing the power button, and watching for the power light to turn green, you hit the "escape" key, then the "5" key, the command prompt is listening forthwith (sometimes called the "PROM" but that is ambiguous). That is where "unsetenv monitor" is applicable.
The IRIX root prompt, however, is only available after the system boots, and stores its settings on the root partition, not in NVRAM on the mainboard. So using "setmon -x 60HZ" to change IRIX's graphics setting is not going to work unless IRIX has been installed to disk. It will not work if you have simply netbooted the kernel: there is no root partition.
You can, however, change the graphics setting for the current session by issuing the "setmon -n 60HZ" command to the root shell prompt. This should at least let you see if the graphics output is working with your display.