Have you considered using the LiveUpdate Administrator software?
I've listed a couple of articles below for how you would go about hosting definitions on an LUA in an isolated network, as well as how to configure unmanaged SEP clients to use this internal LUA location to get their updates:
http://www.symantec.com/docs/TECH106254
http://www.symantec.com/docs/TECH166129
http://www.symantec.com/docs/TECH123388
Is this more what you were after?