Not sure if it would work, but wouldn't it greatly complicate creating a backdoor when the host's memory is very heavily constrainted? e.g. an AVR-based "computer" that would hardly do much more than actually translate assembly to machine code?
It would probably be easier to detect, but when Thompson deployed the attack in practice, nobody noticed, even though the PDP-11 address space was the same size as an AVR's, and the CPU was a lot slower. (AVRs invariably have much less physical RAM installed than a PDP-11, but you'd need some external memory to get a C compiler to build.)
