@Brian Bradburn, based on your description (“the instant the thrower’s foot hits the goal line the disc was thrown”), I believe the existing 18.2.5.3 applies and prohibits such a maneuver, regardless of whether or not we add a ground check.
18.2.5.3. anytime the thrower must move to a specified location, the thrower does not establish a pivot point before a wind-up or throwing action begins;
[Annotation: After a pull, a turnover not in the central zone, or after momentum causes a player to leave the playing field, the thrower cannot take a run up to make a pass whereby they start their wind up or throwing action before they have established a pivot point.]