Apparently, California draws a distinction between a Constitutional Amendment (which Prop 8 was) and a Constitutional Revision. My understanding is that a CA Constitutional Revision requires passage from a Constitutional Convention, not a ballot initiative. If repealing exisitng rights from the CA Constitution legally requires a Constitutional Revision rather than a Constitutional Amendment, then, perhaps, Prop 8 can be found to not have properly changed the CA Constitution. Since the CA Court already found that the CA Constitution contains marriage equality rights, then Prop 8 would be unconstitutional.
Not an expert opinion, but it sounds like a pretty clear path to a challenge. I still hold out hope.