There's a functional character creation menu, there's a command in the code somewhere to open it. At its most basic all the mod needs to do is put a button somewhere into the game that lets us run that command in the code and we're all gucci. There might be other complications (like the current creation menu leading directly into the start of the game, for instance) but I'm certain there's a way to figure that out. Also, there's code for changing appearance in the game already in the form of the shapeshift/disguise self spell, so its not as though there's zero precedent for this. Really, it's just a matter of time until somebody figures out how to mod it in cleanly. A better question IMO is why Larian didn't add the feature themselves - they seem to have accomadated pretty much every other factor of character customisation, including class respec (Something you don't even usually see in tabletop 5e) but didn't for one second think that maybe, just maybe, players would want to edit their characters once they see them in game? Baffling, honestly. They even have an NPC who would be perfect for the job in Withers! It's a no-brainer, so it not being there is the strangest part of all of this.