Superball, unless I missed it, or this is what you meant when you referenced AAU, you omitted the following exception from the VHSL, rule 27-8-2:
"The foregoing provision does not apply to recreation programs
sponsored by governmental agencies and civic organizations when approved by a member
school. In approving a program, schools shall insist that the program be open to everyone
in the community or areas served, and the program not be limited to varsity or junior-varsity
athletes from a specific high school."
It seems that the above rule permits a high school coach to coach in a Little League program, American Legion program, AAU team, etc. as long as it is not limited to athletes from a specific school but it does not appear to have any fractional limit such as 1/3.