I've just tried it against r87935 and it applies cleanly.

Perhaps you didn't apply the patch correctly (it requires "-p1" since it was a Mercurial diff), try:
   patch -p1 < sendfile_v2.patch

With regards to the different arguments, I don't know if that's acceptable or not or if there is a better way. Since you can have mmap.mmap() with differing args between Windows & Unix, maybe it is acceptable. And, Python exposes differing functionality via the posix module since the available functions differs widely between platforms.