Having posted the above message, I then connected my Apple to double check.
And yes you can not "received" audio master signals via the BeoPort+Apple ...
... except ... (drum roll please) ...
... by fortuitous accident I triggered a B&O Wireless IR-eye with my signals, and suddenly the BeoPort's BeoLabs were playing music from the BeoSound audio master
To make this work, on the Beo4 I press N.Music which activates Itunes and the BeoPort and the BeoLabs - I then press STOP on the Beo4 which halts iTunes playing but leaves the BeoLabs active and I then press CD or Radio or A.MEM and I can access the BeoSound 4's sources
A bit of a kludge, and I now have a set-up that looks like this:
BeoLab 3s ==v
Apple === BeoPort ==||
||= Master Link === BeoSound 4
B&O Wireless ==||
So in explaining the issue I have found a workaround (at the cost of a B&O Wireless)