[VALJ]

Casebook 10.4.6.B is the closest I can find to this play.

B1 intercepts A's pass in B's EZ, B2 clips A9 at B's 10-yard line after the change of possession. B1 then fumbles in B's EZ and the ball rolls out of the EZ and out of bounds at B's 2 yard line. RULING: The penalty is enforced under the all-but-one principle. The end of the run is the goal line and the basic spot is the 20-yard line. The penalty would be enforced half the distance, fista and 10 got B on the 5 yard line.

It's not quite analagous, since in the casebook play the penalty happened in the field of play, but it reinforces that the foul in this case is enforced as a spot foul under all-but one.

Though actually, I think somebody quoted this rule earlier, and I can't see why this wouldn't apply:

10-5-4: If the offensive team throws an illegal forward pass from its end zone or commits any other foul for which the penalty is accepted and measurement is from on or behind its goal line, it is a safety.

B's become the offensive team by gaining possession of the ball, and committed an illegal forward pass from the EZ. Right?