It was formally added 2012-05-30 .. go to the Google Code page for it and update.
I have no idea how well it works. I tested against Linux and a Synchronet BBS run by a friend. But I've not heard a single report on it (bug or otherwise), which is unusual. As careful as I am, if somebody is not finding a bug then it probably is not being used.)
Ah! I'm pretty sure I have the latest build of mTCP, so I guess I must have it then I'll dig through the docs and figure out how to use it and report on outcome. Thanks!