I have not tried the shifter on the accessory T bar, but I did try a shifter slightly aft of that location, but oriented differently. First photo, I use the 55mm T bar to hold my handlebar bag bracket. I then put a mountain bike handlebar end on that T bar and put my shifter on that bar end.
Second photo shows the road conditions that I rode for a 14 hour day in 2016.
On that 14 hour day, there were several hills that I was climbing and I had to have both hands on the bars for better leverage and control, but being on a hill I wanted to downshift but did not want to take my hand off the handlebar to downshift. And about half way through that day I decided that when I got home I would buy the Hubbub adapter and mount my shifter on the right side of my handlebar. Then I could have my hand on the handlebar for leverage and also shift gears without moving my hand off the handlebar.
The Hubbub adapter comes in two lengths, if my memory is right, the Rohloff version is shorter. I think I bought the Shimano one and cut it to Rohloff length, but that was six years ago so details are long forgotten. But one thing I do clearly remember, I read that some people found the hubbub adapter could come loose from the bars so I installed it REALLY tight with threadlocker on the threads to make is as permanent as I could because if I put some leverage on the bars, I did not want it to work loose. It has remained solid.
I used a couple V brake noodles to change the angle of my cables coming out of it so the cables are farther from my knees. See third and fourth photos.
Very happy with that setup. That said, I have bar end shifters on three of my other bikes and have used bar end shifters off an on since the 1980s, so it is natural for me to reach down there to shift gears. If you have not used bar end shifters before, that position might not work as well for you, at least not at first.