As far as the sliding spreader bar goes, once you move the pivot point right next to your body, I believe the sliding part is no longer needed or useful. Just picture the angle of the trim line when you are on toe side.
Intriguing hypothesis... here is how I plan to test it... try hooking up first to the old school "slider rope bar" ring and then, the same day, the same wind, the same board, I will then snap the shackle on the stationary center ring (which I just rigged up, per your suggestion... thankyou!).
It will be a "double blind" study, as I learned in my statistics class, is the most accurate kind of hypothesis testing. The test results will show whether or not the "null hypothesis" can be rejected... which stated is: "There is no difference detected"
In order to best make the powerline go to the toeside positions, I plan on jumping from "blind to blind" using, first, the rope ring, and then switch to using the stationary ring. So the test should actually be a double "double blind" study... I will have to check with my old statistics professor for confirmation of this making for an even more accurate test study....................I can't remember the professor's first name, 'cause we always just called him Professor W, but, the last thing I recall him saying to me was "Go fly a kite!" ... I'll make you proud of me, Prof W!
Sooooo, here is the series of hopefully, self-explanation pictures, showing the "test subject bar". I don't remember where I got the "flat bar", but by using it, the balance point of attachment is brought even closer to the body than with a conventional round spreader bar.
Also, note that this "flat bar" could easily be rigged up with a secondary quick release, like the one I showed a picture of, using the old "needle" release (slide-bolt action), and also note that a spreader bar, set up like this could also give the kiter the choice of using the center ring for an activity like "tricks" and then using the rope slider ring for an activity like "wave riding", even switching rings while the kite sits on a wingtip on the water.... AND, it is hard to imagine a bar configuration, that would give a kiter.... more "throw" on the power line. Also, note that the smooth, flat surface of the "flat bar" presents no objects, which could accidentally trigger the primary quick release. And... bla, bla, bla...... lets just hope the bar is strong enough and doesn't bend... ha, ha...