I've not looked at the Wordpress code for generating URLs in a very long time, but I don't know how they could possibly automatically know to generate SSL URLs for every request in the setup you have - the requests are *not* coming in as SSL as far as the web server hosting the code is concerned.
I've set up an identical infrastructure before, but I use Nginx rather than Apache, so I can't be much help on the specifics for your implementation. That said, you took the time to set up an SSL concentrator in front of presumably a cluster of individual web servers, so you're definitely a non-standard user. I'm sorry I couldn't provide you the exact line to include in your config, but it does strike me as odd that you'd go through all that effort and then not be willing to spend 10 minutes testing to find the right line to add to your config.
Sorry we didn't live up to your expectations, we'll try to do better in the future.