trigger server_data event from within client_data

There is no way to do that. SERVER_DATA is only triggered when the server sends data and there's a TCP collect on the server side. You can't be guaranteed to start that collect in CLIENT_DATA because there may not be a server side connection yet. The best way is to set a variable in whatever event you are in when you know you need to collect server data, and then do something like:

 
 when SERVER_CONNECTED { 
   if { $need_to_collect } { 
     TCP::collect 
   } 
 }