This seems to be something prone with the possibilities of major damage to your system — I'm merely pointing this avenue of research out as raised by another webpage, with the preface that you shouldn't even approach doing this until you are quite confident in the knowledge of what you are doing!
This webpage speaks of how the command nvram, located in the /usr/sbin directory, can be used to "list and set open firmware settings," including "specify[ing] your own settings. For example, you could create a variable called asset_tag and keep the asset inventory tag of that particular computer in the firmware."
This particular tip was thought of by the webpage's author as a hint for people running computer labs, but it could be used by other Mac OS X users, I'm sure.
I'd really, really pause and make sure you know what you're doing before goofing around with your computer's firmware, though ... talk about playing with dynamite!
[Editor's note: I'll echo the sentiment -- this is published here mainly in the interest of completeness. Do NOT mess around with Open Firmware unless you really really know what you're doing!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020129204215863