Every now and then open enrollment is offered.
It was offered in January and June/July in 2010.
An alternate is to see if a group has formed here in the Families part of Disboards for one of the homeschooler groups (you don't have to be a homeschooler). The discounted tix are still good for the kids tix (and are substantial), but I think only one adult per five kids is eligible for the adult tix.