I'm receiving ISM ("InSimMulti") packets from the relay that identify as "guest" (for the "Host" field) and contain an empty "Name" field. Reading the InSim.txt file, this seems to indicate someone is joining a game that's not in multiplayer mode. Is this a correct reading? Or did I miss something somewhere?

ISM is for identifying the server, not anyone on it. If you get guest and especially no name field, that says single player to me.

Quote from Stuff :

ISM is for identifying the server, not anyone on it. If you get guest and especially no name field, that says single player to me.

So packets from a single player instance of LFS are being sent to the relay at isrelay.liveforspeed.net and it's in turn sending that to the connected relay clients? Or these packets are being sent when a new relay client connects? I guess I'm not sure where single player instances of LFS relate to the relay.