kqueue is a _huge_ interface, combining a number of ideas, you basically need to design your kernel around it if you want users to use it. And much like SYSV streams, expecting other people to reimplement a giant redesign like that is often expecting _way_ too much.
On the other side, timerfd(), socketfd() and epoll() are all there to do a single thing. FreeBSD did the retarded thing when they reimplemented sendfile() roughly a week after Linux added it. TCP_CORK is still not implemented in FreeBSD. mremap() was NAKd, there's the whole MMAP_ANON vs. MMAP_ANONYMOUS, or the weird bits of mmap in general.
Cooperation is much more like to happen in the FreeBSD => Linux direction, IMO ... but that could also just be the much bigger developer pool.