OS X Serial Number in VM
I have an OS X client VM within VMware Fusion.  It does have a serial number (starting with VMW...).  This is different to the host Mac.   

VMware Fusion seems to generate the serial automatically, but it is not obviously stored in the vm's configuration files.  Also not mentioned in the vm logs.  My guess is that it is 'burnt' into the .nvram file when the vm is created or copied.

As you know the UUID can be changed, but the only references I can find to changing the serial number relate to downloadable OS X vm images.  For example: Change serial hack  This supports my guess in the previous paragraph.

Legal? Using downloaded images breaches Apple licenses - but you are not doing this.  Serial number: Not so clear as it does need to be globally unique - it would be bad to change it to a value used by another vm or physical mac.

But, I don't understand your need to change the serial.  VMware will make sure it is unique if you copy vms to create development and test environments.