It seems that contrary to documentation that is floating around in various articles about NetBackup fileshare source, you don't necessarily have to configure and use null shares for UNC paths. In looking at this the other day, I quickly decided to add the domain computer account for the server to the permissions on the share that I setup for the LiveUpdate path. Windows will, by default, try to use the computer domain account for any localsystem initiated outbound traffic for UNC if the computer is part of a domain.
The result is that now my servers that are part of our domain can utilize our NetBackup LiveUpdate server via UNC file share path without null shares being setup. As we only have a few "appliance"-type servers that aren't bound to our domain, they are just handled manually...
I know for sure this works on 2008 and 2012, not completely sure about 2003, but it's worth a try before implementing null shares.