That is probably the single biggest theoretical roadblock to proper phasing that I can think of. In the example you list then I'm pretty sure the only truly viable way to handle it would be for you to simply vanish from Tad's perspective or simply for you to never appear to one or the other in the first place. If you are in a party with one of them then you share a phase and if you are in a party with neither then you get shifted into the same phase as one but not the other. With lots of "friends" and "ignores" it could potentially be confusing with lots of isolation but then the solution defaults to "be in a group if you want to share a phase" and then there is no more problem.
I think phasing is the wrong word for it, but maybe you can call it dynamic population loading (hah!). Essentially each zone or area loads people based on a hierarchy. Let's say an area is designed to hold or support 40 people. So the game needs to make multiple copies of the zone once you get to 40+ people. So instead of the people in each zone being arbitrary and also forcing you to instance hop until you find a good one, you create a social hierarchy.
The game client/server would have to record people you interact with. So once you enter in an area the game will put you into an instance copy with people that match this criteria:
Friend List
Guild Roster
People you've whispered in the last 10 minutes.
People you've whispered in the last 2/4/8/10/24 hours
People you've grouped with in the last 48 hours.
People you've whispered in the last 48 hours.
People you've traded with in the last 48 hours.
People you've been in combat with or near in the last 48 hours.
People you've grouped with in the last 10 days.
People you've whispered in the last 10 days.
People you've traded with in the last 10 days.
People you've been in combat with or near in the last 10 days.
Random people
The 48 hour number is arbitrary, it could be anything.
You would put an extremely high emphasis on those in your guild and friends list. Then you would populate the region with as many people that are connected with each other. Hopefully the regions can hold more than 40 (again it was a random number, make it 1000) people though. This way you also get rid of servers as well. The more you interact with people the bigger chance you can create your own organic community.
There would be areas that have higher population caps so you're always introduced to newer people. You can always send whispers to anyone in the game. This way if you communicate with a friend, new player, you will have a very high priority to be around that person. You can then create real instanced areas in cities for taverns to meet up with specific people to do trading hand to hand or whatever. If you meet up with someone in any of these instances they go to the top of your priority list.
The more you're around people, the higher likelihood they will show up in your circle of interactive people.