It's possible to do so. In short legacy connection factory must be created on EAP 7 server. For this is necessary to create new acceptor and connector on some port(5445) to which EAP 6.4 server will connect. Also you will need to open remoting port for JNDI lookups and add legacy jndi name to your queue.

Here are CLI commands I used to do that on EAP 7 server:

/socket-binding-group=standard-sockets/socket-binding=messaging:add(port=5445)
/socket-binding-group=standard-sockets/socket-binding=remoting:add(port=4447)
/subsystem=messaging-activemq/server=default/remote-acceptor=remote-acceptor:add(socket-binding=messaging)
/subsystem=messaging-activemq/server=default/remote-connector=remote-connector:add(socket-binding=messaging)
/:reload
/subsystem=messaging-activemq/server=default/legacy-connection-factory=HornetQConnectionFactory:add(connectors=["remote-connector"], entries=["java:jboss/exported/jms/HornetQRemoteConnectionFactory"])
/subsystem=remoting/connector=remoting:add(socket-binding=remoting)
/:reload
/subsystem=messaging-activemq/server=default/jms-queue=MyQueue:add(entries=["java:jboss/exported/jms/queue/MyQueue"], legacy-entries=["java:jboss/exported/jms/queue/LegacyMyQueue"])

# disable security as it's not needed
/subsystem=messaging-activemq/server=default:write-attribute(name=security-enabled,value=false)
/:reload

Lookup from EAP 6.4 server can be done by:

  final Properties env = new Properties();
  env.put(Context.INITIAL_CONTEXT_FACTORY, "org.jboss.naming.remote.client.InitialContextFactory");
  env.put(Context.PROVIDER_URL, "remote://127.0.0.1:4447");
  context = new InitialContext(env);
  ConnectionFactory cf = (ConnectionFactory) context.lookup("jms/HornetQRemoteConnectionFactory");
  Queue queue = (Queue) context.lookup("jms/queue/LegacyMyQueue");

Yes, with this configuration HornetQ Core protocol will be used. There is no other (supported/tested) option for this use case as AMQP and STOMP protocols were removed from Artemis from EAP 7. However Artemis as standalone messaging provides them. Also A-MQ 7 which is product based on Artemis will support them.