I agree, I can't think of a safe way to accomplish a transfer like that.  You can't change the address the hodl'ed coins are tied to until they unlock, and until you did that the seller could move the coins at will when they unlock.  You'd be rolling the dice that the seller would not be waiting to swipe the coins back as soon as they unlock. 

You could put the payment in escrow until the coins unlocked, but if that is the case the seller should just wait till they unlock and then sell them.  I do suppose you could lock in an agreed price by doing that, but again, since the seller can't put his coins in escrow because they are locked, it would be totally up to the seller if they wanted to go through with the transaction at the time of unlocking.  If the price of Hodl goes up dramatically between now and then, the seller would likely just bail on the sale.

The devs will need to build in some additional options for handling locked coins before a sale like this is possible with any degree of safety.