You've convinced me that the dynamic selection of backstory-related tags, as the game progresses and the player chooses dialogue options, wouldn't be such a good idea. Indeed, PCs that are lying or are amnesiac would pose difficulties. To which there are two main solutions. One is to just let the player know that secretive or amnesiac PCs simply aren't supported. Another one is to not use this method. Which means : mini-questionnaire at character creation is the way to go ... Possibly. I want to emphasise 2 things though.
Firstly, this questionnaire should really be minimalistic. The first 3 examples I gave are probably sufficient.
- Place of origin. The list should include one entry for each place for which there exists content (i.e. at least one person, somewhere in the game, who has previously met your PC). If the player writes any "NameOfPlace" of their choice, then it means they won't meet anyone known to the Pc, they'll just be able to have this place written on the character sheet and have the PC say "I'm from NameOfPlace".
- Previous experience with adventure or danger. This could perhaps allow for slightly different party dynamics. Maybe your companions would be more impressed if you prove surprisingly adequate in adventuring when your PC never did this before. And maybe they would be more ready to defer to you if you've let them know you have some experience (well, Astarion and perhaps Shadowheart).
- Age. This could give access to various exclusive dialogue options. Or ... just not exists at all. Well, it would be nice on the character sheet, even if it has no impact.
It is completely clear that Larian cannot (and should not) acknowledge in some form, and provide reactivity for every backstory that players could possibly come up with.
Secondly, all this is really
if some backstory elements have to be used at all. And I don't think that, for a Fully-Custom Character, the backstory should be a main point of focus. What should count about the PC is who they are and the choices they make. What they are made of, what they really value, and who they become.