Update the rule to enforce both feet behind the marker regardless if on the ground or in the air and this debate never comes up again.
So then follow through on a 400' approach shot would be a penalty, assuming you still must plant within 30cm? Sounds like S&D, not a fan of that rule change.
Or are you saying both feet behind the marker until the disc is released, then do what you want? In that case, it would still be an issue of determining if that happened or not.