The exploded view in my Haynes manual shows that the shaft of the side button shifter is angled and that the top of the rod it pushes against is angled. So when you push in the side button it slides against the vertical angled rod to push it down.
If you can find a push down button shift knob that will attach securely and engage the rod to push it down you should have no problem. I have seen several ball type shift knobs that had a button on the top but I have never tried to see if one fits. I was also interested in doing this but never got around to doing this as I was given a 4cyl T-5 for my 2.3L project.