I had this same issue several years back. Loved the 9000, but wanted the cassette function of the BS4000 as well, in the same room. I tried all sorts of ways to ML them up (with the Avant as well in the room). but the fact that there were two audio masters in the same room on the same wire confused the hell out of the system (when CD function needed to be accessed).
Tim, it didn't work for me... but that's not to say that your mileage may vary with the use of the Beo5. When I visited the dealer this past saturday, he was using a Beo5 and on the screen had all the product's names, so I gather that the Beo5 can somehow differentiate between them? I don't know, as I don't have any experience with it.
What I can say with pretty good confidence is that when you have a Video Master, and 2 audio masters on the same ML wire, the 2 audio masters are going to compete for attention. After all sorts of option programming, including not having the Avant in their "family" of ML connections. I gave up, even tried having the BS4000 set up as a Link room product for same room operation, and that didn't work.
An viable option would be for the Ouverture to be in a separate room, and the BS9000 in the main room connected up with the BV7-40.
I'd be keen on hearing opinions from those who have made the "two masters in one room" work.
-Sal
Love B&O, but no longer addicted.