its actually pretty tough to sneak on. they check a lot at balbo and columbus. your best bet would be to sneak on around 47th or some other place in between balbo/57th or balbo/hollywood. if you have an old race number you could fold it around your top tube. i usually do that and all they do is glance at it to make sure i have it, but they don't look at it too closely. the numbers for bike the drive are those fiberglass styled race numbers you apply with safety pins. even as a marshal they make me get a number and be official so you know they check.

or....hang around balbo right after the start and grab the inevitable number drifting around the pavement.