I believe eminent domain trumped property right in most if not all cases. The property right "inherited" from Mexico is just property right. Eminent domain is for public good. A court can always grant easement access to the coast to the public using eminent domain.
Last time this came up on KQED, I think that Khosla's lawyers were saying something about being amenable to an eminent domain argument, under which CA would need to compensate Khosla for the access. Evidently, the argument that the Coastal Commission approach has made does not require compensation.
