By no means guaranteed to be unique, but recent versions of the VM 
provide a sessionIdentifier for this purpose, which as you suggest 
could be combined with an IP address and/or process ID. The session 
identifier is currently used in the FilePlugin to help identify stale 
file handles, but the intent is to provide an identifier that more or 
less represents the current invocation of the VM along with the image 
that is was loaded from the image file.

The value is obtained from Interpreter>>getThisSessionID.  Have a look 
at the file plugin, or if you have OSProcess loaded, do "OSProcess 
accessor getSessionIdentifier" to find its current value.