Karoline,
I agree that identifying the outcomes of a STEM endorsement are an essential first step. I would state that a STEM endorsement would certify that a teacher can teach trans-disciplinary and integrative STEM courses, map critical junctures of science, technology, engineering, and math in curricula, and effectively coordinate and collaborate with teachers in a variety of STEM disciplines. Those qualities would differentiate an integrative STEM endorsement from existing endorsements in math, technology, and the sciences. To be able to teach and coordinate integrative STEM curriculum, the teacher or coach would have to have experience with content, practices and pedogogy from the various STEM disciplines, curriculum mapping skills, and experience in integrative STEM.
Scott Greenhalgh