All, as my own little contribution to the OTP, I've successfully mashed together the MagTrack merger piece with a Hot Wheel coupler. I have printed it out and tested it successfully and it works like a champ. My son absolutely loves it!
I will say when running from MagTrack into Hot Wheel, cars tend to make the transition better if you use the slightly higher walled HW tracks that come in a couple of the sets, especially if the cars have picked up some speed.
The STL file can be found here on Thingiverse: https://www.thingiverse.com/thing:4326045
Please forgive the tape and paperclips, my printer ran out of filament so the top pieces never got printed. Even in its unfinished state though, it works pretty darn well!