... from my brief reading about USB C JTAG on newer Intel systems, the answer is probably: it's all very well knowing where the JTAG connection is, but actually using it is only possible with additional Intel hardware and software, which is going to remain out of the financial reach of most non-corporate developers. Unless anyone knows more.
JTAG is a widely used common standard to program and debug microcontrollers. There are lots of JTAG tools from several vendors, one well known is J-Link. I have several JTAG tools and hooking onto a microcontroller is fairly easy (5 wires needed).
In the case of the MacPro, JTAG on the LF is wired to the SMC controller. This means we can hook up a programmer / debugger to the Hitachi H8S (currently Renasas) SMC to flash it. Now reading the SMC is another story, you can bet the SMC is read protected.
Hacking a protected MCU in general is far from easy and a deep knowledge about the MCU is required. Methods involves
side-channel attacks where glitches are used to destabilize the MCU into an unknown state where memory can be read. This is not the path we want to go.
On the other hand, the MCU can be erased and programmed, communication between logic board SMC and CPU board can be examined, so theoretically someone could develop a program with the same functionality as currently used by Apple (controlling fans, providing information to the CPU, detecting fault states etc). Still not easy but I’m not saying that it’s impossible.