This is exactly what airlines like LH do. They block a few seats at the front of the Y cabin for gold card holders and allocate seats to most people who buy tickets. Once all seats are allocated or blocked then you cannot allocate a seat at booking time (hopefully late booking gold card holders still have some free seats to choose at the front blocked part).
About 25h before the flight they release the blocks and start to sort out what to do including moving people from Y->C (sometimes done well before you get to the airport). In the worst case you will check in at the airport and not receive a seat and they will sort things out at the gate once check in deadline is reached. You will get on if someone fails to turn up or they ask for volunteers and do a VDB.