BeOS, OS/2, Commodore, etc. (For Linux, there is a separate topic dedicated entirely to just that!)
Get some fine gauge wire, like wirewrap wire, and solder one to the pad for C97 that has a via about 7 mm away. Solder another wire to the trace from pin 35 on the CPU (you'll need to scrape the solder mask off the trace). Then connect the other ends of those wires to a 1.8V to TTL level shifter IC which you'll connect to an RS-232 driver like the MAX232 or maybe just use one of those logic to USB serial converter boards. Then you can fiddle with the baud rates and, if you catch it at the right time, you can't interrupt the boot sequence via the serial terminal.