Thank you, if you write that up as an answer on SO I would be happy to accept this there too. I'll write up a bug on the issue coreWCF issue tracker over the weekend.
The more I looked into the issue, the more it felt like I was trying to do something WCF isn't designed to do.
I don't have an SO account, but I welcome the kudos here!
My experience with Microsoft-related stuff now being done "in the open" on GitHub has been great, because it means that users like us can directly open an issue ticket right where the developers are day-to-day. The tickets get attention and/or fixes more more often than screaming into the void on some generic forum full of only vaguely interested people.
The more I looked into the issue, the more it felt like I was trying to do something WCF isn't designed to do.