The game I'm working on for this game jam is planned to be a much bigger game than what is feasible for my to complete by the deadline. Is it okay for me to get the first chunk done and post that as a sort of demo for the eventual full game? 

Do like a title screen, a level, and an end screen. (make sure you can get to it with some sort of goal, even if it's temporary for the jam) You'll be a lot more satisfied if you get something finished than if you don't.

As long as you've got a playable demo, you're good to go! We're definitely looking for quality over quantity here, so just use your best judgment for level of completeness. No matter how long it takes you to finish, we'd still love a link to the finished product--no matter if the jam has closed.

I am doing a visual novel and the story I eventually want to tell is far to long so I am making it one section at a time with each section being developed during a different game jam over the next few months.