That's an interesting idea. While it is true that magi from another House could take MMF in Certamen, it is hard to imagine most players ever doing so. Certamen is so limited as a Focus that few players would be willing to restrict their growth in this way, unless, of course, that was the entire point. A player who picks MMF: Certamen is asking for a lot of Certamen in the game, and as a GM I'd like to give that player what he wants.
That said, I always try to discourage PCs taking virtues which another PC got for free; it really waters down the torture of Tytalus apprenticeship when another PC from a more friendly House has Self Confident. If I had a Tremere PC in my Saga, I would ask no other player to take that Focus.
In other words, while it's technically possible, I don't imagine I'll ever see a player outside of Tremere with this Focus. If it ever appears, it would be in an NPC made specifically to challenge a PC Tremere on his home ground.