Client Certificates at the Backend?

Try this on a virtual server with an attached SSL and HTTP profile:

rule foo {
   when HTTP_REQUEST {
        if {[SSL::cert count] > 0} {
            HTTP::header replace SSLClientCert [b64encode [SSL::cert 0]]
        }
    }
}

This BASE64-encodes the client certificate, if any, into an HTTP header called "SSLClientCert", replacing it if the client already has a header by that name. You'd need to BASE64-decode it on the back-end, and then you'd have the original certificate again.

Let us know if this works for you.