That is what I like about using custom launchers. I am using Prism Launcher [1] which keeps all instances (mod packs for me) locally. It includes the MC version, Forge and all mods. If it gets too big for my main partition, I can just archive and move the instance folder to my NAS. I can launch an instance "offline" which doesn't seem to connect or auth with anything. I have never tested though what happens if I do not have a recent-ish auth session.
Bonus feature: I do not have to deal with the Curse client.
I am running prism but I'm not sure that helps if you want to play on a private server (that has disabled auth checks, like I'm sure a bunch of private servers will do if Java dies officially) after the auth servers are shut down.
Edit: anyway, you don't have to deal with the curse client if you're comfortable with installing plugins by hand. Or so it was before i switched to Prism, i could download the jars off Curse without their client.
