Which obviously defeats the purpose of using it without Internet. Perhaps caching our log in info? I can imagine there might be a "hacking" solution to get round it. But on local servers...is that really an issue? 

^^ 

Comment accepted. Surely if clearing a zombie is a chore then perhaps you're missing the point? The joy is surely not in human conquering but in intelligent scripts that can do so with minimal losses. 

Would a simple login timer not work? 

Whilst yes a good automated script will run indefinitely until killed. An active player does at least check on their screeps from time to time. Give you a month or two to check on your screeps else your scripts stop running?