Bits is bits - so sorry you couldn't figure out the right bits to make the hardware do what you want, but those bits do exist and they are findable in principle - it's still all software! Your lack of knowledge of the signing keys is qualitatively no different than your lack of knowledge of any other undocumented proprietary information required to program the chip. Frankly you'd struggle even with the keys.

Infeasible? Oh my yes. Impossible? No. That's the point - there is a distinction.