You can have a custom party if you play with 3 friends, so I would think you would be able to do the same in single player. If you can't, I would consider that an oversight that should be fixed before launch.
The original BG1/2 allowed for a fully customizable party for network (multiplayer) games and they had their own save location. The trick was to start a local network game, create your party, save then copy the multiplayer save to the single player save location, then you could play single player with a fully custom party. The caveat there was you'd occasionally have to boot a custom party member to include a narrative forced party member (or if you wanted to do some NPC sidequests)...and sometimes those custom members would just disappear forever. There were other tricks and mods to deal with this, but that's the gist of how it worked then. The Enhanced Editions just integrated full custom party creation into the single player game...and booted custom party members would just wait where you left them if you wanted to re-add them.
Possibly a similar trick will end up working with this, though obviously impossible to know at present, short of a dev speaking on this.