Presently there is no way to set custom end-points for the steering servo (apart from doing it in your transmitter, which as you point out has unintended consequences in the case of halftracks). This was an oversight on my part.
The steering servo is not the same thing as the barrel elevation servo, they are unique objects in code. It's true the steering servo is routed to output 4 in some cases but it doesn't have to be (it will be output 2 if you have a car or halftrack with single rear wheel drive). Setting the end points on the barrel elevation servo is not the same as setting them for the steering servo.
I will work on adding a servo-setup menu for the steering servo to give the user the ability to set custom end-points on the servo object specifically.
In the meantime, the best thing you can do is what RCers have been doing for decades - move the steering pushrod closer to the center of the servo control horn, and farther out on the steering control horn. I will recommended this no matter what firmware changes I make. Full steering movement with only 35% stick movement is a little silly. Even when I create a way for you to adjust the end points to compensate, doing so will come at the sacrifice of resolution (ie, fine control). Setting up the mechanical control to utilize as much of the servo travel as possible is the first priority, afterwards we use end points and other gimmicks only to fine-tune.