Extracting client IP at the pool member

It would be a fairly simple iRule.

when HTTP_REQUEST {
  HTTP::header insert "Client-Addr" [IP::remote_addr]
}

If you don't want to go with the iRule route, you can always enable the "X-Forwarded-For" header option in the associated HTTP profile. This will insert the client address into a "X-Forwarded-For" HTTP header. You'll have to use the iRule if you want a customized header name.

-Joe