Yes, is possible to get into the 'BIOS' (OpenFirmware), but its well nigh impossible to set it up right
Firstly, you need to figure out what port on the back is Serial 1. Its usually the Modem GeoPort. Then you need to get a convertor lead to convert that to a standard DB9 serial.
Connect that serial to another computer or a serial console, set up a serial communication programme like HyperTerminal to 9,600/8N1. This will let you into the 'BIOS'.
Now, power on the machine whilst holding Apple-Option-O-F. This requires three hands, or a very big reach one two hands.
If you're very, very lucky the HyperTerminal will then pick up the 'BIOS' settings terminal.
Note that you can completely bugger the system up doing this.
