On some systems it is possible to get the serial number using ipmitool. This is because on most motherboards that support ipmitool general system information is accessible. Finding this using ipmitool is helpful since a system may be down and not easily accessible, but you still have access to the remote access card and can query via ipmi. In order to access the serial number one can use the ipmitool command as follows:
ipmitool -H <host> -U <username> -P <password> fru
The ipmitool fru prints built-in FRU and scans the SDR for FRU locators. This will often result in a great deal of system information, one of which may be the system serial number.