Yea holepunching is implemented in the dht-rpc package. There is an explanatory comment there as well, https://github.com/mafintosh/dht-rpc/blob/master/lib/io.js#L... (hope that helps!)
We are planning to blog more about how the internals work. The "first class" hole punching is def one of the features I'm most exciting about.
I am not sure in which part of the code to look for this (is the hole punching part of the dht package?) so any pointers are appreciated