Assuming it's not a legitimate free course and it is left with flags in, I wouldn't want to play... i.e. you'd effectively be stealing golf. It it was Augusta, then maybe I'd be tempted to go and play 9 holes as I certainly wouldn't have a moral issue with stealing golf from Augusta.
But if you are that keen to play St. Andrews, especially through the Winter, then just go and play it. It's reasonably accessible and not that expensive.
