OK,  UDP seems really proper with low latency for what you are doing.
The message encryption is done by the publisher which is the POW part, right?
Than the publisher fan-out the message to all known endpoints and they store the hash only?
Do the nodes who receiving the message send them further to all endpoints they know, or the hash only?
(I guess this would be the validation process)

How do you handle serialization of all this many small packages?
Do you use protocol buffer or another queuing library?

Sorry for the many ?. But distributed apps are very fascinating (problems).