You can just have a custom SQLFunction and pass whatever prefix you like as parameter. In your example you'd have a function called my_usr_id that accepts a string literal as argument. The implementation the just uses that string literal during the rendering process. The HQL query would look like this from User u where u.id = my_usr_id('test') and the essential part of the SQLFunction looks like this

public String render(Type firstArgumentType, List args, SessionFactoryImplementor factory) throws QueryException {
  String literal = args.get(0).toString();
  String schema = literal.substring(1, literal.length() - 1);
  return schema + ".my_usr_id()";
}