The only answer I know is the one that is the title of this post. There a couple of examples at least of Intergalactic Championships.
Rick White and Mary Wells have been doing a great job of promoting the Hobie Wave class in the United States and nearby countries. A few years ago they tried to organize a Wave World Championship in the Turks and Caicos Islands. Mary tells what happened next in this thread on catsailor.com.
As far as us promoting it in the United States to get people to go to it, we were under intense pressure from ISAF and from US Sailing to change the name to something that would not imply in any way that it was a world championship. They suggested "Intergalactic," because, thankfully, ISAF has not yet extended its governing authorities beyond this planet. The only reason we knuckled under and briefly changed the name to Intergalactic was because we had a youth sailor coming to the event, and we did not want him to be disqualified from being able to race in ISAF youth events just because of participating in the (illegal) Wave Worlds.Yes, ISAF will threaten to disqualify you from sailing in other ISAF events if you sail in a worlds that is not a worlds!
The other example is the Tech Dinghy, a single-handed class that originated at MIT back in the 1930s and is still actively raced there. There are only a few other fleets scattered around the United Stated as far as I know. They also hold an Intergalactic Regatta, presumably for the same reason.
A quick Google confirms that the same solution has been chosen by the Soverell 33 and the San Juan 23 classes. Don't ask. I haven't a clue what they are either.
There may be other answers to Tim's question. Anybody know of any?
The other thing I don't know is has ISAF ever withdrawn the right to run World Championships from an existing class once it had granted that right? I don't know of any examples. Do you?