Good news, I finished my X-Brake install tonight. I got sick of waiting for the X-Brake people to respond and just went for it.
The bumper sticker version: yes, you can definitely loosen the Torx bolt on the lever arm and rotate the arm about the splines to adjust the start-of-throw position. I moved it one spline counter-clockwise and the clevis fits like a glove. No hacky adapter necessary. Takes less than 5 minutes.
Since the instructions on their website are for the old brake and super confusing, I will give you some hints here.
First off, to install, just pull everything off first. Pull the t-case side of the rear driveshaft, remove the brake drum, remove the shoes and springs, and the backing plate from the back of the t-case. Remove the linkage and the linkage bracket from the side of the t-case. You only need to save the clevis pin and its split pin.
If working from a creeper under the truck (feet towards front of vehicle), you'll find it helpful to put your right arm over the passenger-side frame rail to get at hard-to-reach fasteners.
When you fit the brake and bracket, have a look at my photo here to see how everything is supposed to look. The instructions mention a technique where you bolt one side up and rotate the brake caliper around the disc. I didn't have to do this. I basically just put the caliper around the disc and bolted everything up and it was fine.
Chances are good that the cable--I replaced w/ the one they sent--is much too short to attach the clevis pin with the caliper arm in the stock throw position. To remedy this, you will need to loosen the T45 Torx bolt securing the arm and rotate it counter-clockwise one spline. Put some good work gloves on and hold the arm so that you're fighting the spring slightly and loosen the Torx bolt slowly until you can unset the arm from the spline. Your caliper assembly will look different from the one in the instructions but the 5mm of loosening is about right. The arm will wiggle free and you can carefully rotate it counter-clockwise to the next spline....maybe 15-20 degrees? While continuing to hold the arm, re-tighten the bolt. You should feel the arm bed down nicely on the splines if you have it right. Once this is done, the clevis should fit perfectly. I attached mine to the middle hole of the lever arm using the clevis pin from the old brake assembly.
Once the arm is re-tightened and clevis installed, make sure that the big bracket bolts/nylocs (22mm... 3/4"?) are tightened and the bracket is secure.
Next, you'll need to adjust the two 19mm nuts on the end of the cable to get the throw just where you want it. I started out with mine in the center of the adjustment range and ended up tightening it a little bit from there. The way I adjusted mine was to put the rear axle on jack stands and put the t-case in neutral. I laid on a creeper with my foot against the rear tire, which I used to spin the driveshaft. I would adjust the throw a little and then spin the tire with my foot to see if the X-Brake was dragging. It did make a faint dragging sound even when disconnected from the clevis. Basically, I got it to the point where it was almost braking and then slacked off a little from there. I did some tests in my driveway. 5-6 clicks of the brake lever to get to fully secure is where I am now. I might tighten a little more after some road miles.
This next photo shows you the barely-visible splines on the caliper shaft. They are just barely peeking out behind the rightmost coil of the spring.