Well, there was the placement of Blackmoor in John Sniders space campaign as a planet - which is where you get the Avains and the whole City of the Gods business, and how the crossover from Tekumel was possible. There's also a legendary story Arneson wrote in 1974 for the Bel-Ran newsletter about the destructive "med-a" (media?) in which a place called Blackmoor and Bleakwood are mentioned as being on a planet called Urth. While the story is a kind of quasi religious history, the name of the planet could be taken as a basic fact if you like.