RE "MantaRay is unique in that it uses a server-less, purely distributed architecture." - this is not really true.
ActiveMQ supports a pure peer based architecture too; its the default out of the box configuration in fact. You only need to run brokers if you wish to manage persistent files on certain machine - rather than having to worry about backing up 1000s of machines persisted messags.
So Mantaray is hardly unique - other open source JMS providers support peer networks