(1) If he doesn't have the reverse servo there should be a hole left in the chassis. Another servo mounted next to, and inverted, like steering servo might be able to be linked together so two servos are pushing the same drag link. End point adjustment and linkage adjustment would be critical so the servos wouldn't fight each other.
(2) The other option would be to move the receiver box and cut a hole for a second steering servo. (Like the Revo). End point adjustment and linkage adjustment would be less critical. Each front wheel would have its own servo and they'd need to be trued up for tracking and turning.
The above ideas would require some good parts and/or chassis machine work. Got a mill?
I would just buy an HS5645mg steering servo.
[edit]
And what about servo savers? If you don't use the stock Traxxas servos with built-in servo savers then option one could allow one Pro-Line steering kit. If you went for option two with non-stock Traxxas servos... well.. I don't wanna think about engineering that...