"easiest/low tech" would be some sort of unix file-locking and inode. Only works if you are on the same host machine though...AAAND sounds like it would be quite hacky.
Which is a good thing :) I am all in for asking questions. If someone can't answer them, it usually means they have not thought about it and are either ignorant or will learn something themselves :D
(like me for your question, as I never tried TabSub with internet disconnected. Was delighted to see it works nevertheless)