This was specifically addressed at our interpreter's meeting here in the state of Washington (after several calls to the NFHS for clarification according to our interpreter).
Here was what we were told (so don't shoot me, I'm only the messenger). LGP has to established with both feet in bounds. Once it's established, the defender can draw a charge with one foot out of bounds.
So player B1 establishes LGP against player A1. A1 is dribbling near the sideline. B1 moves to stay in front of A1 and in doing so, steps out of bounds with a foot. A1 charges into B1. Player control foul on A1.
Z
|