A message popped up on the Apache modules-dev list which was a follow-up to a post I made way back when about using digest auth with proxies AND different paths. The spec seemed to suggest it was possible, but I couldn't to get it to work properly. I just wrote this back to the list for what its worth:
If this is indeed a follow-up to my question way back when about mod_rewrite, mod_proxy, and digest authentication, I was unable to find a real solution. What I ended up doing was excluding the digest authentication from proxying, and instead doing it on the forward facing server (the one doing the proxying).
This worked because the back-end pages were accessible without authentication, but they used http_auth values to identify users for non-apache based authorization.