Maybe this sound a bit substrus (?), but when I had a mac mini connected to my BV7-40 I used a Philips Suround headphone amp HD1500. This device converts digital audio - DD, DTS, PCM - to analogue stereo including passthrough of digital sound. So Mac mini digital out -> HD1500 digital in -> HD1500 analogue out (headphone out) .> Scart audio in BV7 AND HD1500 passthrough digital out -> digital in BV7-40. Now you have digital sound, no matter whether DD, DTS, stereo, AND analogue stereo for the linkrooms. Simultaneously and everything in lipsync. No need for changing or switching connections, no need for the Cambridge converter.
BTW: you can`t distribute video via masterlink.
Hope this helps
Stefan