Not only that the arbitrators must be familiar with the rules and practices of the project in question. It is not enough to read the languages.
That's an excellent point. I think that pretty much rules out a meta-arbcom considering cases for small projects without their own arbcom.
I don't even know of any instances where the en:Arbcom has ruled on any problem arising in one of the other English languafe sister projects. If it tried to impose en:wp practices on any of them I'm sure there would be a storm of controversy.
The enwiki Arbcom doesn't have jurisdiction over other projects, so would never accept cases from them. The only way they would ever end up considering a case for them would be if they were approached by the community of that project, so there wouldn't be any controversy, since it would be that's project's decision. I doubt the enwiki Arbcom would agree to it even if they were approached though, it's not what they're there for.