Not sure what you are up to, before even creating a game one could pop up a message "Warning: stun enabled, random success, please consider forwarding, read more ...". Connection testing can also be done by the lobby, upon creation of a game or otherwise, which I consider a low traffic solution. The important bit is to educate game hosts that stun is a poor solution in general, the indicator is mostly for people to learn if they aren't able to connect to stun hosts so as to get rid of the impression that it's 0ad fault.