You did WHAT with iptables? (Part 4: The "Revert to old behavior" Solution)
I was conferring with my newest colleague, and explaining to him the hack job I had to do to make music on hold work. Apparently, his google-fu is better than mine, as he found this gem:
Turns out, they did put in a "revert to old behavior" option. It's called "Duplex Streaming Enabled".
Needless to say, that works much better than what I was doing.