To be more clear, the missions I am talking about are when the player speaks to their Gods, for example. I would have assumed that every character would have this content available as the player could chose any race/build when they started. It feels like the Tags that you mentioned are not applied because they weren't created at start.
That happens at a specific point. If a character was an NPC at the point you did that, and a player took control over them after that, then no, they don't get to see that event because it has already been finished. You would need to load a game from before that point and have the extra party member take over there.
Would it be easier/weirder if the request was a way to create a Avatar Character I can inject into a save file?
That's flat-out impossible. Characters cannot be separated from save files.
