Just a heads up for those that want to try this before it’s an official feature- you can make a custom party by starting a LAN game with yourself.
Open the game directory (browse local files on Steam), open the bin folder, launch the game from the exe as admin. You can do this 4 times so you are running the game in multiple windows. Start a LAN game with one and join with the others. Create a character in each, venture forth, and close all of the BG3 windows except the host.
Ah ha ha ha, exact same as DOS 2.
That's really helpful, considering that I really, really wanted to wait for a 4-character custom party, so I didn't get burned out from seeing all the origin stories over and over again. Thanks a lot.