Sending a file descriptor along a socket?

[Christopher Lemmer Webber]

It looks like ffi/unsafe/port provides ways to read a file desciptor
from a port that is capable of receiving them, and there are tools
to transform that file descriptor into a port or the reverse, but
is there a way for me to *write* a file descriptor to a port
(ie, the equivalent to the sendmsg posix system call)?
(It might be right in front of my nose and that I'm missing it...)

Thanks!
- Chris