Does anyone know if the the new W5700X MPX module (or stock GPU for that matter) allows for hardware encoding in OBS or other streaming software, similar to NVENC for Nvidia cards in PCs?
OBS has no idea and no control which chip is going to do the hardware encoding. It only knows if HWAccel available or not. If yes, then it can command the macOS to use it.
I am not a OSB user, no idea why it doesn't show up on your cMP. However, your argument cannot lead to the conclusion that OSB is using the T2.
I see from your signature that you also have a MP6,1. Are the steps required to activate VideoToolBox hardware encoding on the MP2013 basically the same? I have a trashcan with 2xD500 and an eGPU with RX580 or Vega 56.