I heard last year that they were going to allow two, non-3 event senior slalom drivers at the Western Regionals. I don't know if it's happened or how it's worked out.
The rule Jody posted has been around a while. I'd say that mostly it works out that way. If more end course video was set up and available, I think most drivers would benefit. In the mean time, the boat judge is in charge and has the responsibility to correct boat path, and the authority to grant re-rides for out of tolerance boat paths- even in non-record tournaments... Communication between boat judge and driver is huge.
That said, giving a good pull is not just about the times and path. Two drivers with near perfect paths will rarely feel the same, or 'equal'. Steering input, drops, pull-ups, etc.- all affect the skier, and influence their confidence in the driver, and their skiing.