Sneep is the way to go.
Until it gets moved to the oracle website, you can get it at http://www.sun.com/sneep ,
or as a part of the Service Tools Bundle at Sun Services Tools Bundle .

On newer hardware sneep can generally find the serial using any of several kinds information available from the system.
On old hardware, there might not be any way to get the serial for you.

See HardwareSerial - SNEEP - Serial Number in EEPROM - wikis.sun.com

You said that you have Netra equipment, and while there are some fairly recent Netras, most of them are getting a bit on the old side, so you might need to actually visit the machine to get the serial the first time.

Once you have it, use sneep to save the serial into eeprom, and you will never have to look at the hardware for it again.

sneep -s actual_serial_number

Sneep will also preserve and protect otherrelevant information that might be of use to you, such as data center rack location, service contract, asset tag, etc.

Since the sneep data prints when the system is booting (if use-nvramrc?=true), I have even used it to remind the system administrators of the alternate break sequence on machines which are configured to use it.

sneep -t ALTERNATE_BREAK -s "is configured: use return-tilde-control-B to break"