I don’t know if that’s as big a problem as you think. Assuming they have not previously allowed special characters at the beginning of Twitter handles (I don’t know whether they had the foresight to do this), they could use a character that was disallowed previously and is a legal character in URLs and then include that special character in the URL path so that it would be twitter.com/~user/ or whatever. This would only be applicable to new official accounts and would not break URLs for existing users.