When the goal of homeschooling is meet the needs of gifted kids, it's probably easy to forget that 2/3 of homeschooled kids are there for religious reasons (see
this article, for example). This means that those kids probably won't get exposure to ideas their parents don't like. I've known people in this group, and they can be hostile to ideas about evolution, vaccines, and sex ed. I tend to see the last topic as a civil right (a right to know how your body works).
Parents of all homeschooled kids should have to fill out registration paperwork and submit a 1-2 page syllabus for each class. A syllabus is a good thing to get parents to think ahead, and if the kids are enrolled in homeschool groups, the groups presumably make those documents anyway. My son and daughter did a few of these classes (locally run in person), and they all had syllabi. CTY certainly has them.
The kids should also take annual bubble tests. They'd be easy for the gifties and would give wonderful credibility to the parents and homeschool groups. They'd also identify the students mentioned by pinewood1 who are being neglected or abused.