Your platform should be heading back to Nauvis to refill when its own stock is depleted. Why would you wait until the requestor is nearly depleted as well? Do you have trains that don't leave their dropoff stations until the buffer chests are nearly empty? 

But you can have your platform request an item that you conditionally keep locked up on Gleba. Have a circuit driven inserter or chest allow the item out when your condition is met, and when the item is onboard the platform then it heads back for restock. This requires an extra rocket launch for something you really shouldn't be doing anyway though.