Earlier quoted context omitted.
> N/RVO works by (at the machine language level, of course) rewriting the function signature to return void and take an extra pointer parameter This sounds wrong, are you sure? Would you mind demonstrating with an example on godbolt? Whether NRVO applies or not, the ABI should be the same, AFAIK.
Yes, of that I'm sure. This optimization is only possible if the compiler has control of both sides of a call. If the function may be callable from other translation units or modules I imagine it generates a thin wrapper that's externally callable.
I don't know why I wrote the comment about parameters earlier -- (N)RVO is about return values.
Those have a different reason for being passed behind a hidden pointer: the class type might have self-referencing pointers, so there must be an explicit move/copy constructor call whenever it changes address, to give the class an opportunity to update those pointers. This cannot work when returning in a register: the callee doesn't know the target address, and the caller doesn't the source address, so neither can call the move constructor.
Thus, all ABIs must pass a pointer (or let caller+callee agree on a memory location in some other way) for types that aren't trivially copyable.