I'm not used to UDK, by the way I'll try to answer your question at just a theoretical level. I'm trying to implement network games in a project of mine, and found out how to let client ping check servers without actually joining games. Also, this is an intuition from Halo:CE network code.
Let's suppose the central server browser has a list of servers, these info are ready to be sent to requesting clients. When a client wants to browse servers it actually only gets a list of IPs to check for: for every IP, the client sends a data packet to that IP (which is the address of a server currently running a game) with a data message to request server info, such as server name, game type, map name, no. of players. Also, client sends its timestamp attached to this packet.
When a server receives such packet, it sends back another packet with the requested info: server name, game type, map name, no. of players; then CLIENT's timestamp from previous client's packet is attached to the new packet.
When client receives server's response back, it can show in a browser window these info, plus round trip time (RTT) to that server, which is: RTT = current_timestamp - old_timestamp. You don't need to store old timestamp, because you're getting it back from the server itself.
All this can be done without having a client joining a game by using UDP connection protocol: you just have to set up a different port for "ping" information exchange, and set up some raw packet messages to be recognized both on server-side and client-side machines.
HCE works this way, I guess: client only asks central server for a list of IP, then every single IP is sent a packet to get information about current game running and ping value relative to the client. When refreshing servers list, clients ask server browser back for a new list of IPs to check for.
FYI servers can use two ports: a port client_port (this is no code, just a generic variable name) is used to sync the current game with connected clients; then another port server_port listens to incoming packets from clients which want to ask server "Who are you? What game are you running?" and answer these clients back.