Client Certificate is not passing through back end hosts

is this not a use case for Proxy SSL feature?

It is indeed, but a) you didn't mention ProxySSL in your original question, and b) ProxySSL comes with some pretty significant caveats.

As with any SSL "man-in-the-middle"-type solution, ProxySSL requires knowledge of the server's private key and an RSA-based key exchange between the client and server. This last requirement is proving harder to achieve as the industry moves away from non-perfect forward secret cryptography.