I love the write up. I also like your thinking in the creation of you homemade middle of the road controller.
Had similar ideas in the past myself but never used the 700r4 long enough to fine tune things.
I would like to mention that you have test port plugs on the transmission that lead to certain fluid circuits in the trans. Without getting my books out to see which ones you may have on your model year... You may want to look and see when each one's get pressure during certain conditions.
You may find one that can work an oil pressure switch that may be activated when vehicle speed drops.
Another thought is downshift from 4th to 3rd calibration...
Of course, you could add a speed sensor and use a little electronic engineering to operate a switch or control under certain conditions
- Outside my abilities to design, but understand electronic engine controls theory.
Once again, love the work your doing.
Bob