The main problem is that you have to put back the same amount of money, and stocks change value. If you withdrew 10 shares of stock at $10, you can't reverse that by sending back the 10 shares of stock if the price is now $10.50 (excess contribution) or $9.50 (not the same amount of money going back as came out).
I agree that the plan trustee will have the ultimate say, since they are audited on what they allow you to do. But I suspect the answer will be cash.
