Rules!? Us?!
This isn't a rated jam, so we don't have a problem with you using the time to continue an active project, especially if you think it works well with one of the jam themes. We'd just ask that you let everyone know what you had going in so we can see what you did during the jam. Oh, and don't work yourself to death! Your work is yours to continue after the jam to, of course.
I'll add a spot in the submission form for you to mention any pre-work.