If there isn't a way at the moment, I recommend that NetApp implement something a la Solaris' ``eeprom'' command, which allows modifying OpenBoot environment variables, and is really handy when you don't want to take a machine down to change stuff, or you can't get to the console due to unforseen problems (such as when you terminal concentrator locks up that port :( ).
RFE 14368 has been filed on this.