There is a third problem.
If the genetic material sequence in the GMO is patented, the information cannot be divulged by a third party certification.
One has to have the genetic ‘print’ in order to even test for it, and those that posess that data are not likely to self-incriminate, so that may not ever be an issue.
The thrust of the labeling requirement is to disclose the systematic use of known GMO. Ballot propositions are constitutionally limited in scope, partly for such reasons.