I have just been reading through the rest of this thread – some great detective work! The requirement for pins 3, 5 and 7 to be connected is indeed strange!
One thing to note from this test is that the BM5500 is the only product where I have found that the data pins in the Aux connector and the MCL speaker connector are connected internally in the Beomaster, so the result of this test might not be the same with other audiomasters.
I think BC9500 is the same (TV AUX pin 6 connected to MCL speakers). So the data going over an MCL system is Audio Aux Link (AAL).