iRule for port translation of single public IP address to multiple real server IP

I wouldn't recommend doing this. However, maybe something like (completely untested):

when CLIENT_ACCEPTED {
    TCP::collect 3
}

when CLIENT_DATA {
    if { [TCP::payload length] >= 3 } {
        switch [string range [TCP::payload] 0 2] {
             list out the first 3 letters of the HTTP methods we want to see
            GET -
            POS -
            HEA -
            PUT -
            DEL -
            OPT {
                 looks like it'll be HTTP
                pool http_pool
            }
            default {
                 doesn't look like HTTP
                pool https_pool
            }
        }
    } else {
         we didn't get as many bytes as we wanted
        TCP::collect [expr {3 - [TCP::payload length]}]
    }
}