I was thinking an FPGA with an open toolchain might make a pretty solid hardware seed that can fairly easily be replicated, without needing your own fab or anything completely impractical. In theory the FPGA could still try to recognize and backdoor a bitstream right when you load it, but practically there's a whole bunch of different open-source cores that you could load on the FPGA, and it would be very hard to try to recognize and backdoor all those potential designs at the netlist level, or at least it would take a suspiciously large area, potentially visible as clearly more complicated than what should be needed on a microscope.

You could maybe distribute these verified boards that are just the FPGA and the verified CPU core + kernel + software seeds all stored on a basic NAND flash. But then instead of writing a lisp interpreter in machine code as your seed.. the seed would have to be an entire basic CPU that runs something like hex0, or like a basic lisp machine. You'd plug a keyboard into it and bootstrap the rest from there. But good luck with that, I'm not volunteering to try to make all of that work!

Plus there's still a small chance of a hardware backdoor somehow powerful enough to recognize any possible core you can load on it, or that the tools you use to even load a NAND flash or the NAND flash itself could be backdoored, so really we're just displacing the problem a bit further every time. But if you want to bootstrap everything from nothing, then you basically have to jumpstart all of computing electronics back up from nothing.

(And yeah, this is fun to think about!)