Antoine Pitrou: "In any case, I think this would take the form of new low-level calls in the posix module, i.e. add thin wrappers around the new system / libc calls."
"new system / libc calls"... well, preadv() is available on FreeBSD since FreeBSD 6 :-) And preadv() and pwritev() first appeared in Linux 2.6.30; library support was added in glibc 2.10. It's not really something "new".
It seems like the two functions became more interesting for Python since Linux implemented the new RWF_NONBLOCK flag. aiofiles can be used to bypass its thread-pool, for better performances.