> supports seeds and peer discovery, message passing, maybe firewall / UPNP support, etc.

Our audacious 'IPv8' work is now deployed live. Might fit you needs. Includes above features in a low-complexity approach. https://github.com/Tribler/py-ipv8 Our fresh IETF Internet Standard draft of this work: https://tools.ietf.org/html/draft-pouwelse-trustchain-00