If you are connected through the serial port via Telnet (e.g. through an Annex or similar device), you can use a Ctrl-] to get the Telnet command mode, then type "send break" at the prompt.
This will also stop a booting system, and allow you to configure OBP stuff.
I personally have three Sun Blade 2000s at home - one is wired to the monitor, with a keyboard and mouse, and the other two are wired in to my Annex, so I can still log in and control them from either my PC, Sun workstation - or a wireless-connected notebook.
(I'm sure the power company loves me, too.)
