How's this:
It would require a few code changes though. 1st, the 3 convergence turrets on the wingtips would have an extremely narrow field of fire, and their normals would need to be aimed precicely at the nose of the ship. 2nd, the table entry for those beams would make its beam range so short that it would terminate exactly at the nose of the ship. The fire duration would also need to be made to last just longer than the main beam, so to give the power-up effect. 3rd (coding required), the fire-beam sexp would need to be altered to include an option for the beam being fired not to target at any specific ship, but rather to fire precisely down its normal. Then, its just a matter of timed beam-fire commands between the wing-tips and the nose. There'd also need to be an event to render the fire sequence inoperative if any of the 3 convergence turrets are destroyed.