I don't think there really is a problem. Simply practicing with a JC is OK from an eligibility point of view. (It's even OK to practice with a pro team. 12.2.2) The only thing that is concerning is that the NCAA lists the ways (14.5.2) a person is considered to be a transfer student, and one of those ways is A transfer student is an individual who transfers from a collegiate institution after having met any one of the following conditions at that institution: ....(g)The student participated in practice or competed in a given sport even though the student was enrolled in less than a minimum full-time program of studies;.
I think that person who is still a high school student won't be considered to have transferred from a college (bolded section above), even though they were receiving dual credits, or even registered in a non-dual credit JC course.
So my only concern is with a home-schooled student, who might have trouble showing that he was really in high school. Sounds like he can show that he is still in high school, because he hasn't graduated yet.