> What needs to happen to get recvmsg() supported in Python?

Well, I guess that the only reason is that no committer is motivated
enough to bring this into Python: it's a rather large patch, and
honestly, I'm not sure that many people are going to use it.
The feature I personally like the most about sendmsg/recvmsg is the
ability to do scatter-gather I/O, but if the performance is critical,
then I won't be using Python.
I know that sendmsg also has some other advantages (passing FDs,
ancillary data...).

> recvmsg() is required to get get transparent UDP proxies working under Linux
> using tproxy, the code needs to run recvmsg() to be able to find out what the
> original destination address was for the the packet.

Sounds like a job for raw sockets, no? (well, you need CAP_NET_RAW)

In short, I think that you just need to find a core developer
interested, I personally am not (but I'm not opposed to it either :-).