Glibc seems like a good place to put it to me. If everyone is preferring hand-coding assembly primitives that workaround suboptimal standard methods (ucontext),

Glibc seems like a good place to put it to me. If everyone is preferring hand-coding assembly primitives that workaround suboptimal standard methods (ucontext), then my first instinct would be to look into making those existing methods more optimal. (Note: in case it wasn't clear, I'm not suggesting we put all of coroutines into glibc, just the stack dancing stuff)