When there is no keyboard present, most SGI machines switch to serial console on port1. Just hook it up to another box (Unix Workstation, PeeCee running Linux/Windoze, doesn't matter) with a serial "nullmodem" cable. Start a serial terminal (e.g. minicom on Unix/Linux, HyperTerminal on Windoze) and set it to 9600,8N1,hw (9600kbit, 8 bits, no parity, 1 stop bit, hardware flow control). Power the machine on and you should see the PROM messages.
If that doesn't work, hook up a monitor and a keyboard to access the prom via graphics console. Go to the "Command Monitor" and enter "setenv console serial(0)" to set the console type to serial port 1. After that, enter "init" and you should see the prom on your terminal. Now you can remove the monitor and the keyboard.
From now on you can install Linux or maintain your box from here.
Some machines like the Indigo2 need a minidin8 -> db9 adapter.