This is a question for the plan administrator to answer as this could be tricky or not allowed. 

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.  

If an RMD is taken "in kind" as in shares of a stock distributed then that exact same stock must be rolled back - you cannot substitute cash or another stock to roll it back.    The rollback is to restore the IRA to what it was before the distribution was made with the same shares of the same security.

This is defined in tax law and not but the plan administrator.   In addition the IRS Pub 590A states the same thing dealing with rollovers.