This is going to be a relativity short update. It's 1:30 in the morning, but I finally got portals to work!
It's not perfect. There are a few issues I need to sort out:
1. You can see that there's a flash right at the moment when you walk through the portal.
2. You have to be facing the portal in order to go through it (can't walk backwards into it).
3. You can't carry objects with you through the portal.
4. Still need to set up portals to teleport between different Unity scenes.
I'm not too sure how to approach these issues yet, except for the last one. I believe I can pull off portals between Unity scenes with additive asynchronous level loading. I think the problem of carrying objects through the portal will be the most difficult one.
However, pretty happy to have gotten portals working. They're going to play a big role in the game.