That's harder thing to do on every iteration down to the first principles. So in theory you maybe could do the hammer, which is made so as to specifically clog nails with a deviation of 5.2 degrees to the left, which leads to the light bulbs distortions in the chip designer room which eventually leads to particular backdoor in the chip he designed - but this is really hard.
The example in Trusting Trust was a very specific case: he modified the C compiler to replace a known bit of code in the login program. Along those same lines, you could possibly set up your minifab such that it inserts a backdoor into a particular RISC-V implementation. However, if I sat down and made my own chip, how would the fab figure out how to insert a backdoor? If you have code which can analyze a processor layout and seamlessly insert a backdoor, please come forward and collect your Turing Award.
You are aware that making a chip with any reasonable processing power either requires using existing designs, or teams of hundreds of people for several years, yes?
You're back to trusting trust. Or using toy computers.
That's why we are talking about cheap small mini-fabs instead of just placing the order to any Chinese fab. The mini-fabs would use open-source community-driven software which is easy to audit for backdoors. That's not easy, but the trend is toward that, for sure - e.g. pick and place systems driven by OpenPNP is a step in this direction.
