I'm leaving Milwaukee on an international journey, changing planes in Minneapolis for arrival in Japan. Can I meet a friend there in Minneapolis, leave the checkin area, or will I be "quarantined" in some international holding pen?

You can leave the secure area, but you'll need to pass through security again. You'll need to have your boarding pass and passport to get back to your gate, etc. FYI: I don't recall any services outside of the secure area at that airport, so if you were planning to sit down for a cup of coffee or something, you are going to be out of luck.

Unless things have changed drastically in recent times, I doubt you will be in any "holding pen" - you will get off the plane and wonder around the airport without a problem. And contrary to what Jim says, my recollection of the airport in Minneapolis is that they were a fair amount of food areas where you can grab a bite, just like any major US airport.

Jim & Val...Thanks for info.

There are a lot of places to eat and grab a bite, but I think they are all inside of the secure area. I generally only use MSP to change planes too, but I think that may mean that the friend isn't going to be able to get inside without a boarding pass and ID -- so if the friend isn't flying, that's not going to be a meeting place.