It's not UDP but I do TCP hole punching here: https://github.com/robertsdotpm/p2pd and every other major method of NAT traversal.

It's written in Python. Though its not based on using the default interface like most networking code. I wanted the possibility to be able to run services across whatever interfaces you like. Allowing for much more diverse and useful things to be built. Its mostly based on standard library modules. I hate C extension crap as it always breaks packages cross-platform.