Seems like Rube Goldberg approach to spying
There are tons of legit chips on the board. Instead of adding one which will be certainly discovered sooner or later, why not use one of the chips that are already there?
Just fake a vulnerbility in one of the legit chips, and if discovered, then it is just a bug, like so many others. Make it a hardware bug like Spectre or a well hidden firmware bug.
To me, adding an extra chip to do spy is a Rube Goldeberg solution to spying. The same goal could be achieved easier, with less chance of discovery and with better deniability when discovered. Just don't fix one of the vulnerbilities discovered during development of the board. Maybe even hide it a bit better. And if discovered: no biggy, because bugs happen - to everyone.