I'm gradually working my way along Through the Aegis, and ran across Master Carles' plan to create a Tribunal of the North Seas. Obviously this wouldn't be a concern for the Grand Tribunal of 1228, but it might be for the Grand Tribunal of 1261.
Considering that there is a tradition of perigrination for lone magi, and at least two other travelling covenants, and it may be that a case could be had to found a Tribunal At Large; for covenants and magi of no fixed abode.
The founding covenants would be Northern Seas, Favonius, and Atsingani, with at least one more needed to make four covenants. The Tribunal At Large would have to emphasize mobility:
No covenants could have a fixed location in the mundane world as their council site.
A land travelling covenant must relocate at least once per year, and no fixed vis sources on land may be registered.
Individual Magi-at-Large may not remain guests of a single covenant for more than (three? seven?) years, or otherwise qualify for residency in another Tribunal.
A covenant may hold fixed locations, as for example the shipyard of Favonius, they simply may not have them as their council halls.
Other possible members: Waddenzee; Tres' Band; Semita Errabunda; a covenant of itinerant Redcaps (who only meet on the road); travelling players and minstrels wandering Western Europe; a caravan covenant in Africa; a mercenary covenant wandering all of Europe, especially the Rhine and Transylvania; a travelling camp of air-bourne magi who fly from mountain area to mountain area; a floating castle not necessarily under the mage's control; a covenant of migrating Bjornaer on a fairly fixed circuit.
House Mercere might back it; House Tytalus might as well, for the potential clashes it would cause; House Tremere maybe, especially if the covenant of mercenaries was dominantly Tremere magi; much of House Bjornaer on migration principles.
I doubt a Tribunal At Large could actually form, and if it did it might not stand for long, but it might be a plot thread for someone.