Re: Occam's Razor
a highly complex and detectable method
Depends on who you want it undetectable (or at least nearly undetectable) for. On the manufacturing side you actually need just a few people who know the details: the ones modifying the schematic and the PCB layout, and creating the manufacturing manifests for the board etching/sandwiching/populating machines. It's the ones that handle the boards after they've shipped (building them into systems, reflashing, further inspection, etc.) that these mods need to be hidden from.
Also, for everybody else in the manufacturing chain these are just like any of the other customised boards destined for a particular customer.