Have a member dedicated for softlaunch/test

Just for clarification, do you mean a pool or a pool member? A separate pool, or separate VIP as Steve says, would be easier.

when CLIENT_ACCEPTED {
     collect the default configured pool for the VIP
    set default_pool [LB::server pool]
}
when HTTP_REQUEST {
    switch [string tolower [HTTP::host]] {
        "localtest.domain.com" {
             if request to test server, and is local IP, and test  pool is up
            if { ( [IP::addr [IP::client_addr] equals "x.x.x.x/x"] ) and ( [active_members localtest_pool] > 1 ) } {
                pool localtest_pool
            } else {
                 otherwise send to regular pool
                pool $default_pool
            }
        }    
        default {
             send all regular traffic to assigned pool
            pool $default_pool
        }
    }
}

The above would trigger the IP and active_members evaluation if the request was for the "localtest.domain.com" host, assuming multiple host names resolved to the same VIP address. All other traffic would be routed to the VIP-assigned pool.