I think Broker territories with a gateway is the way to go.
Territory for the 6.5 Broker
Territory for the 8.0 Broker
Gateway between them, specifying the allowed doc types desired to be shared
I have seen this arrangement work (between 6.5 and 7.1.2).
I’m not aware of any other way for Brokers to interact with each other.
#broker#Integration-Server-and-ESB#webMethods#Universal-Messaging-Broker