Consider the domain concept as a security realm, this means that each domain has its own user/security space. All actions on the "bus" have permissions related to them, some are open but most require some sort of authorization level. However, a "post" from on hub in domain A to another hub in domain B is possible, through a static route (defined in the sender hub). The receiving hub may choose to block out the request through the Access List mechanism.
So the short version is: You can point a queue from one domain to another, but you can't perform any command requests etc.
Carstein