If you don't raise the $2500, they will not provide you with the rider ID tags/package, so you won't be able to participate. My first year raising was relatively easy, but the second time people just felt like I was hitting them up for cash. So I donated the difference to make the $2500.
I did know someone that rode along unofficially, he was asked to stop by organizers, but they could not stop him since it's not a closed route. However, he did not use any of the official rest stops. But I would not suggest or advise any one to do this.