Not sure I understand what you're trying to say here, this sounds exactly backwards to my reading. A green threading library can be written in assembly language with no reference to anything but the hardware ISA specification and the ABI that defines the relevant calling convention. It's as "low level" as a software construct can be. The only lower level implementation I can think of are hardware-assisted context switching features like you see on some embedded architectures.

Coroutines, on the other hand, are artifacts of the language runtime and depend in sensitive ways on that whole stack. Much higher level.