For the completeness of the thread, as I couldn’t solve these strange network related issues (still a mystery to me), here is finally how I managed to get it working. I simply run the following command from a startup script:
dns-sd -P MediaCenter _daap._tcp . 3689 MediaCenter.local. 10.0.0.2 &
which simply creates a Bonjour proxy for _daap (iTunes shared library) on TCP port 3689, named ‘MediaCenter’, to the ‘MediaCenter.local.’ computer (Bonjour hostname) whose IP is 10.0.0.2 .
Bonjour Browser then lists the service properly, and iTunes can finally play my shared library! 🙂
Wow… why on earth would it route multicast out the ppp connection?
That certainly works, but dang, what a pain. 🙁